enabling explicitly_egress_direct to prevent flooding of packets

In the upstream bug report, a bridge flooding issue with openvswitch firewall driver was discussed, and in the end a new config option in Neutron as explicitly_egress_direct was introduced.
https://bugs.launchpad.net/neutron/+bug/1732067
https://opendev.org/openstack/neutron/src/branch/master/releasenotes/notes/accepted_egress_direct-cc23873e213c6919.yaml

There are some conditions when to enable it so we might want to introduce some logic in the charm to handle it automatically or expose it as a charm config so it can be enabled explicitly.

> A new config option ``explicitly_egress_direct``, with default value False,
> was added for the aim of distinguishing clouds which are running the
> network node mixed with compute services, upstream neutron CI should be
> an example. In such situation, this ``explicitly_egress_direct`` should be
> set to False, because there are numerous cases from HA routers which can
> not be covered, particularly when you have centralized floating IPs running
> in such mixed hosts.
> Otherwise, set ``explicitly_egress_direct`` to True to avoid the flooding.
> One more note is if your network nodes are for networing services only, we
> recommand you disable all the security_group to get a higher performance.