Use list of cluster member in ovn/ml2 agent to directly talk to nb/sb
central services instead of using haproxy lb. ovn-controller agent
automatically monitor cluster member and remove them from list if
they are dead or not reachable. This is better approch then using
haproxy lb.
Change-Id: Icb490225ff34354b3f5821c5f7a54a039091c924