NeutronApiSDNContext does not allow for multiple neutron-plugin-api-subordinates

Simplest option is to just return the context outside the loops. However, [0] seems to have made this explicit. Need to discuss the original intention.

[0] https://github.com/openstack/charm-neutron-api/commit/4ac5e3f71c2594cd9e74b4dcb36f494ba3dff53d

The problem is much bigger. NeutronApiSDNConfigFileContext could also be affected.

But the bigger issue is merging multiple (conflicting?) settings like neutron-plugin and mechanism-drivers:

# relation-get
egress-subnets: 192.168.151.60/32
ingress-address: 192.168.151.60
mechanism-drivers: ovn
neutron-plugin: ovn
poke: "1"
private-address: 192.168.151.60
service-plugins: metering,segments,ovn-router
subordinate_configuration: '{"neutron-api": {"/etc/neutron/plugins/ml2/ml2_conf.ini":
  {"sections": {"ovn": [["ovn_nb_connection", "ssl:192.168.151.65:6641,ssl:192.168.151.32:6641,ssl:192.168.151.56:6641"],
  ["ovn_nb_private_key", "/etc/neutron/plugins/ml2/key_host"], ["ovn_nb_certificate",
  "/etc/neutron/plugins/ml2/cert_host"], ["ovn_nb_ca_cert", "/etc/neutron/plugins/ml2/neutron-api-plugin-ovn.crt"],
  ["ovn_sb_connection", "ssl:192.168.151.65:16642,ssl:192.168.151.32:16642,ssl:192.168.151.56:16642"],
  ["ovn_sb_private_key", "/etc/neutron/plugins/ml2/key_host"], ["ovn_sb_certificate",
  "/etc/neutron/plugins/ml2/cert_host"], ["ovn_sb_ca_cert", "/etc/neutron/plugins/ml2/neutron-api-plugin-ovn.crt"],
  ["ovn_l3_scheduler", "leastloaded"], ["ovn_metadata_enabled", true], ["enable_distributed_floating_ip",
  false], ["dns_servers", ""], ["dhcp_default_lease_time", 43200], ["ovn_dhcp4_global_options",
  ""], ["ovn_dhcp6_global_options", ""], ["vhost_sock_dir", "/run/libvirt-vhost-user"]],
  "ml2_type_geneve": [["vni_ranges", "1001:2000"], ["max_header_size", "38"]]}}}}'
tenant-network-types: geneve,gre,vlan,flat,local

root@juju-206174-0-lxd-6:/var/lib/juju/agents/unit-neutron-api-0/charm# relation-get -r neutron-plugin-api-subordinate:21 - neutron-ironic-agent/0
egress-subnets: 192.168.151.60/32
ingress-address: 192.168.151.60
mechanism-drivers: openvswitch,hyperv,l2population,baremetal
neutron-plugin: ironic
private-address: 192.168.151.60
subordinate_configuration: '{}'