MB-8726: implemented ns_memcached:set_cluster_config/2

Change-Id: I85fd65da02a073fd867ce11ee2ca3969ed6c46c5
Reviewed-on: http://review.couchbase.org/29995
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>