reimplemented ns_config:read_key_fast in terms of search/1

Change-Id: Ie0961532e2d096ae77cae52a6ae8557048187b4a
Reviewed-on: http://review.couchbase.org/44629
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>