Timing issue in controller BuyCar Failover CSIT

The Buycar Failover tests sometimes fail when verifying the car-person items after creation. These items are created via issuing an RPC that indirectly creates the item in the DS. The tests issue RPCs to create 100 items and then does a read to ensure the 100 items were created. However, since the items are created indirectly and asynchronously, not all may have been created yet when the read is issued. In https://logs.opendaylight.org/releng/vex-yul-odl-jenkins-1/controller-csit-3node-clustering-ask-all-neon/73/robot-plugin/log.html.gz, items 298 and 299 were missing however the karaf log shows the items were actually created with timestamps slightly after the read.

The test needs to retry the verify phase over say 10-20 sec.

This also needs to be done in every place where cars are bought, eg Buy_Cars_On_Leader_And_Verify.