MB-11924: added ns_memcached api for CMD_CONFIG_{VALIDATE,RELOAD}

Change-Id: Iaf2ed5c81dab5aec216e67e02707a8ee77599e06
Reviewed-on: http://review.couchbase.org/42011
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>