MB-12564: consider only kv nodes for sample buckets quota check

Change-Id: I141a856aa31f29b6eb6feb27b42dd8a7be9ed3da
Reviewed-on: http://review.couchbase.org/42934
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>