Enable live_migration_events in nova-ovs-hybrid-plug

The flag was added to prevent the source host neutron agents to trigger
a vif-plugged event too early during a live migration. But actually it
can be used in a more generic sense as the code filter on migration_to
binding profile attribute.

We saw too early vif-plugged events from neutron during evacuation in
post part of the nova-ovs-hybrid-plug job. So this patch enables the
workaround flag for this job too.

Closes-Bug: #1971563
Change-Id: Ifd20ece3a4f126da16f077247c2f1e072edb7163

.zuul.yaml

@@ -133,6 +133,9 @@
           "/$NEUTRON_CORE_PLUGIN_CONF":
             securitygroup:
               firewall_driver: iptables_hybrid
+          $NEUTRON_CONF:
+            nova:
+              live_migration_events: True
     group-vars:
       subnode:
         devstack_localrc:
@@ -156,6 +159,9 @@
             "/$NEUTRON_CORE_PLUGIN_CONF":
               securitygroup:
                 firewall_driver: iptables_hybrid
+            $NEUTRON_CONF:
+              nova:
+                live_migration_events: True
     post-run: playbooks/nova-live-migration/post-run.yaml
 
 - job: