MB-12564: only include kv nodes into bucket's servers list

Change-Id: I5c4c706f5d7b62da573837ab8d20b9b72467e6a8
Reviewed-on: http://review.couchbase.org/42873
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>