MB-100: short-cut ns_node_disco:nodes_wanted

I.e. it doesn't have to get serialized via gen_server:call

Change-Id: If7c684aaff6c46eec45a996297c2b72d0faacd6d
Reviewed-on: http://review.couchbase.org/27588
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>