CSIT Sporadic failures - External Networks - SNAT UDP Test

There are a lot of runs that have a single failed SNAT UDP test.
Need to analyze this failure, understand if it is a netvirt code issue or test issue and apply the fix.

Note that most of the runs have these tests passing, so it seems like a race/instability.

Examples with a single failure:
https://jenkins.opendaylight.org/releng/view/netvirt-csit/job/netvirt-csit-1node-openstack-mitaka-upstream-transparent-carbon/370/
https://jenkins.opendaylight.org/releng/view/netvirt-csit/job/netvirt-csit-1node-openstack-newton-nodl-v2-upstream-transparent-boron/354/
https://jenkins.opendaylight.org/releng/view/netvirt-csit/job/netvirt-csit-1node-openstack-newton-upstream-transparent-boron/361/
https://jenkins.opendaylight.org/releng/view/netvirt-csit/job/netvirt-csit-3node-openstack-newton-nodl-v2-upstream-stateful-boron/139/