MB-8503 serializing bucket editing through ns_orchestrator

the original problem: if you edit bucket during the rebalance
on a node other than master, then there will be a configuration
conflict and rebalance will fail at the final stage

serializing bucket editing via ns_orchestrator will guarantee
that configuration changes will happen on the same node and
therefore there will be no conflict

note: introducing new methods of ns_orchestrator will not
cause backward compatibility problems since it is guaranteed
that master node is alwaus running the latest software.

Change-Id: Ifcb820c2c205ff63d82dced5f631239ecb1ab08a
Reviewed-on: http://review.couchbase.org/27828
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>