make sure that all rebalance subproceses are dead before janitor_agent

.. executes ns_memcached:set_vbucket and ns_memcached:delete_vbucket

Change-Id: Id7a5324dfaa7517b107a3d7b0f69a4818a536638
Reviewed-on: http://review.couchbase.org/38514
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>