IPV6 adress lost after ODL upgrade.

IPV6 adress lost after ODL upgrade.

periodic IPV6 RA msgs missing after ODL upgrade from x to y , due to
which ipv6 address lost on all vm's.

Issue: NETVIRT-1248

Description:
During ODL upgrade , if cluster owner changes , the RA thread is not
getting restarted , due to which unsolicited RA msgs are not sent
from ODL.
     On every node-reboot, there is RE-connection of OVS(s) happens.
Once node is UP, MD-SAL replays configuration.
During which each listener can fire at random intervals and leading
inconsistent data store.
    This is observed only in a setup where 150 VNF's,
mulitiple networks, subnets, ...
       The issue is observed only in CLUSTERED-ODL upgrade.The same
issue was also reproduced without upgrade, "By rebooting
CLUSTERED-ODL nodes one after another sequentially".

Change-Id: Ibb45313280b433e2243ed8637ef4bdccc85aa61c
Signed-off-by: nithi thomas <nithi.t@altencalsoftlabs.com>