MB-12564: disallow failing over last kv node

Change-Id: I8eba3577082d87469b9f5c151ecd300a9d7755c0
Reviewed-on: http://review.couchbase.org/42874
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>