unexport ns_storage_conf:bucket_databases/1

Change-Id: I097fd550ce94ab0a1edb73ddcc695d4da0970161
Reviewed-on: http://review.couchbase.org/35931
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>