MB-12564: consider only kv nodes for num_replicas warnings check

Change-Id: If7645bf7e0ec7df3e70790d0e5f06a801951af20
Reviewed-on: http://review.couchbase.org/42935
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>