Attempt to Upgrade OVS to 2.7 on HA Overcloud Topology RDO Ocata

This test is inspired by [1], however it has been done on stable 
Ocata branch 15.0.6 (versus master in link mentioned above.) 
So, it allows after OVS upgrade launch completely functional
VM in overcloud.Obviously, no ovn* packages got installed. 
Same step on Master is awaiting complete functionality of
TripleO QS deployment of Master branch.

:Complete text may bee seen here: