Fix ovsdb node update in clustered environment

Fix ovsdb node update in clustered environment
In cluster environment same switch can be connected to multiple
controller instance running southbound plugin. In the existing implementation
if device disconnect from any of the controller instances OVSDB-SB remove
OvsdbNode from operational data store. In cluster environment that will
break contract with customer, given that device is still connected to
atleast one of the two controller instances running. This patch modifying the
behavior where controller will see if its the only connected controller,
then it will delete the data from operational data store, else it will skip
any clean up from operational data store, because other instances will anyway
update about the disconnected controller manager entry in the managers list.

Change-Id: Iaa043f6632eb2de660b623604e8f8e0938dbc4d5
Signed-off-by: Anil Vishnoi <vishnoianil@gmail.com>