do not allow to query vbucket states during rebalance

this can happen if the rebalance fails and master is reelected and
janitor on new master tries to do cleanup before the dcp connections are properly
nuked.

after this change the ns_janitor_vis:graphviz functionality will not
work during the rebalance, which is OK since this is rarely used undocumented
feature

Change-Id: I7753032dd79b29ee27a8a39fc7a53466ea661ded
Reviewed-on: http://review.couchbase.org/40552
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>