[L2][scale] add a trunk size config option for bundle flow install

We recently tested the openvswitch-agent flow installation performance, we have following two results:

1. the AGENT_RES_PROCESSING_STEP is 100 (default)
We can see the 4296 OpenFlow rules are installed in 20s.

2020-12-02 16:01:32.843 792 INFO neutron.agent.common.ovs_lib [None req-106cf38e-e20a-443e-b1ad-24ab1b7dc7cb - - - - -] Processing 4296 OpenFlow rules.
2020-12-02 16:01:52.698 792 INFO neutron.plugins.ml2.drivers.openvswitch.agent.ovs_neutron_agent [None req-106cf38e-e20a-443e-b1ad-24ab1b7dc7cb - - - - -] process_network_ports - iteration:266 - agent port security group processed in 31.044

2020-12-02T08:01:32.303Z|00967|connmgr|INFO|br-int<->unix#440: 1 flow_mods in the last 0 s (1 adds)
2020-12-02T08:01:33.353Z|00968|connmgr|INFO|br-int<->unix#443: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:33.864Z|00969|connmgr|INFO|br-int<->unix#446: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:34.435Z|00970|connmgr|INFO|br-int<->unix#449: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:34.966Z|00971|connmgr|INFO|br-int<->unix#452: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:35.399Z|00972|connmgr|INFO|br-int<->unix#455: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:35.904Z|00973|connmgr|INFO|br-int<->unix#458: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:36.335Z|00974|connmgr|INFO|br-int<->unix#461: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:36.820Z|00975|connmgr|INFO|br-int<->unix#464: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:37.288Z|00976|connmgr|INFO|br-int<->unix#467: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:37.805Z|00977|connmgr|INFO|br-int<->unix#470: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:38.329Z|00978|connmgr|INFO|br-int<->unix#473: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:38.792Z|00979|connmgr|INFO|br-int<->unix#476: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:39.248Z|00980|connmgr|INFO|br-int<->unix#479: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:39.687Z|00981|connmgr|INFO|br-int<->unix#482: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:40.164Z|00982|connmgr|INFO|br-int<->unix#485: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:40.568Z|00983|connmgr|INFO|br-int<->unix#488: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:40.978Z|00984|connmgr|INFO|br-int<->unix#491: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:41.453Z|00985|connmgr|INFO|br-int<->unix#494: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:41.892Z|00986|connmgr|INFO|br-int<->unix#497: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:42.264Z|00987|connmgr|INFO|br-int<->unix#500: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:42.639Z|00988|connmgr|INFO|br-int<->unix#503: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:43.077Z|00989|connmgr|INFO|br-int<->unix#506: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:43.452Z|00990|connmgr|INFO|br-int<->unix#509: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:43.879Z|00991|connmgr|INFO|br-int<->unix#512: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:44.385Z|00992|connmgr|INFO|br-int<->unix#515: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:44.821Z|00993|connmgr|INFO|br-int<->unix#518: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:45.299Z|00994|connmgr|INFO|br-int<->unix#521: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:45.809Z|00995|connmgr|INFO|br-int<->unix#524: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:46.310Z|00996|connmgr|INFO|br-int<->unix#527: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:46.805Z|00997|connmgr|INFO|br-int<->unix#530: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:47.185Z|00998|connmgr|INFO|br-int<->unix#533: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:47.567Z|00999|connmgr|INFO|br-int<->unix#536: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:47.987Z|01000|connmgr|INFO|br-int<->unix#539: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:48.417Z|01001|connmgr|INFO|br-int<->unix#542: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:48.865Z|01002|connmgr|INFO|br-int<->unix#545: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:49.408Z|01003|connmgr|INFO|br-int<->unix#548: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:49.918Z|01004|connmgr|INFO|br-int<->unix#551: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:50.399Z|01005|connmgr|INFO|br-int<->unix#554: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:50.871Z|01006|connmgr|INFO|br-int<->unix#557: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:51.337Z|01007|connmgr|INFO|br-int<->unix#560: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:51.627Z|01008|connmgr|INFO|br-int<->tcp:127.0.0.1:6633: 16 flow_mods in the 7 s starting 27 s ago (12 adds, 4 deletes)
2020-12-02T08:01:51.771Z|01009|connmgr|INFO|br-int<->unix#563: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:52.285Z|01010|connmgr|INFO|br-int<->unix#566: 100 flow_mods in the last 0 s (100 adds)
2020-12-02T08:01:52.695Z|01011|connmgr|INFO|br-int<->unix#569: 96 flow_mods in the last 0 s (96 adds)

2. the AGENT_RES_PROCESSING_STEP is 1000 (changed)
12222 OpenFlow rules are installed in 6s.

2020-12-02 16:13:41.855 3090 INFO neutron.agent.common.ovs_lib [None req-0eef620d-613e-4485-ad2f-b9380c3fc381 - - - - -] Processing 12222 OpenFlow rules.
2020-12-02 16:13:47.846 3090 INFO neutron.plugins.ml2.drivers.openvswitch.agent.ovs_neutron_agent [None req-0eef620d-613e-4485-ad2f-b9380c3fc381 - - - - -] process_network_ports - iteration:0 - agent port security group processed in 202.523

2020-12-02T08:13:42.299Z|01310|connmgr|INFO|br-int<->unix#1298: 1000 flow_mods in the last 0 s (1000 adds)
2020-12-02T08:13:42.718Z|01311|connmgr|INFO|br-int<->unix#1301: 1000 flow_mods in the last 0 s (1000 adds)
2020-12-02T08:13:43.174Z|01312|connmgr|INFO|br-int<->unix#1304: 1000 flow_mods in the last 0 s (1000 adds)
2020-12-02T08:13:43.698Z|01313|connmgr|INFO|br-int<->unix#1307: 1000 flow_mods in the last 0 s (1000 adds)
2020-12-02T08:13:44.087Z|01314|connmgr|INFO|br-int<->unix#1310: 1000 flow_mods in the last 0 s (1000 adds)
2020-12-02T08:13:44.554Z|01315|connmgr|INFO|br-int<->unix#1313: 1000 flow_mods in the last 0 s (1000 adds)
2020-12-02T08:13:45.050Z|01316|connmgr|INFO|br-int<->unix#1316: 1000 flow_mods in the last 0 s (1000 adds)
2020-12-02T08:13:45.534Z|01317|connmgr|INFO|br-int<->unix#1319: 1000 flow_mods in the last 0 s (1000 adds)
2020-12-02T08:13:45.939Z|01318|connmgr|INFO|br-int<->unix#1322: 1000 flow_mods in the last 0 s (1000 adds)
2020-12-02T08:13:46.388Z|01319|connmgr|INFO|br-int<->unix#1325: 1000 flow_mods in the last 0 s (1000 adds)
2020-12-02T08:13:46.869Z|01320|connmgr|INFO|br-int<->unix#1328: 1000 flow_mods in the last 0 s (1000 adds)
2020-12-02T08:13:47.323Z|01321|connmgr|INFO|br-int<->unix#1331: 1000 flow_mods in the last 0 s (1000 adds)
2020-12-02T08:13:47.834Z|01322|connmgr|INFO|br-int<->unix#1334: 222 flow_mods in the last 0 s (222 adds)

So we can say maybe we should increase the trunk (step) size for the ovs-ofctl bundle installaion. We do not want to add a fixed value because the vswitchd may be in high pressure according to your deployment, the flow install performace may not be as good as you expected. So we could add a config option for users, they can tune the size for their cloud based on the load of the agent, vswitchd and cpu usage and so on.