added initial impl. of /pools/default/nodeServices endpoint

nodeServices allows clients to figure out all services of all nodes
without having to specify any bucket. This might be particularly
useful for niql.

Change-Id: I79bf9dfd82023c7cdd462dbf6c5110f7c0cbb24a
Reviewed-on: http://review.couchbase.org/41575
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>