MB-12564: disallow rebalancing out last kv node

Change-Id: I452d68f057c13b565267736873d685cb2b6e2d8a
Reviewed-on: http://review.couchbase.org/42875
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>