MB-100 Simplify ns_config:merge_kv_pairs.

I removed the case that is perfectly handled by merge_list_values
already. And I'm not sure hitting this case is even possible. In
addition to that I renamed merge_list_values accordingly.

Change-Id: If988d7c1258c4fd07c4c27b3972651f02504487e
Reviewed-on: http://review.couchbase.org/30073
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>