nodes not always seen in operational store

This bug is to track failures in CI.

https://jenkins.opendaylight.org/releng/view/openflowplugin/job/openflowplugin-csit-1node-cds-flow-services-lithium-redesign-only-master/

Nodes do not show up in operational store. This CI test case shows an empty set of nodes:

https://jenkins.opendaylight.org/releng/view/openflowplugin/job/openflowplugin-csit-1node-cds-flow-services-lithium-redesign-only-master/235/robot/report/log.html#s1-s2-s1-t1

In a local setup I saw 2 of three nodes discovered, although on the OVS side all three switches show as connected. Also the controller has 3 active connections (1 for each mininet switch) on port 6633.

There was nothing obvious to me in the log that could be wrong.