CSIT Sporadic failures - tempest.scenario.test_network_basic_ops.TestNetworkBasicOps.test_subnet_details

this CSIT case is failing periodically. one specific example here:

https://logs.opendaylight.org/releng/jenkins092/netvirt-csit-1node-openstack-newton-upstream-learn-carbon/143/archives/log.html.gz#s1-s2-s1-t8

initial debugging looking at the karaf.log, it appears that the
gateway router interface is not being reflected as UP:

dIngressServicesConfigBindHelper | 357 - org.opendaylight.genius.interfacemanager-impl - 0.2.0.SNAPSHOT | Interface not up, not Binding Service for Interface: d2818f02-ecf1-4bad-9d4e-434c7aca76c1

and, because it's not up the FloatingIPListener does not install the flows:

FloatingIPListener | 374 - org.opendaylight.netvirt.natservice-impl - 0.4.0.SNAPSHOT | NAT Service : No DPN for interface d2818f02-ecf1-4bad-9d4e-434c7aca76c1. NAT flow entries for ip mapping InternalToExternalPortMap{getExternalId=Uuid [_value=9a326fe1-5816-4729-a321-f5e95b64a002], getExternalIp=10.10.10.253, getInternalIp=10.0.0.6, augmentations={}} will not be installed