MB-12564: compute balancedness of only kv nodes

Change-Id: I280c5c7ae7077d1458e2977aa0348333dea3b805
Reviewed-on: http://review.couchbase.org/42876
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>