When the external network is deleted the vpn opinterface entires are not removed. This is leading to stale flows and exception in conntrack mode.

[1]https://logs.opendaylight.org/releng/vex-yul-odl-jenkins-1/netvirt-csit-1node-openstack-queens-upstream-stateful-snat-conntrack-fluorine/102/robot-plugin/log_03_external_network.html.gz#s1-t40-k4-k2-k1-k2-k58-k3
[2]https://logs.opendaylight.org/releng/vex-yul-odl-jenkins-1/netvirt-csit-1node-openstack-queens-upstream-stateful-fluorine/562/robot-plugin/log_03_external_network.html.gz#s1-t40-k4-k2-k1-k2-k58-k3