(CDAP-521) App local services

Ability to have Application local services. They are discoverable only from within the same application. 

In addition, the ``HttpServiceContext`` was missing the ability to discover other Services. Added that too.

Depends on: https://github.com/apache/incubator-twill/pull/18
I think the easiest way is to use some object as the payload and then use gson to serialize/deserialize the payload. Default values can be specified in the constructor and you can tell gson to ignore any unexpected or missing fields. I doubt it's the best solution, but should be ok for now. Maybe later we can have some common code that handles this, so we can eventually execute some code that upgrades CDAP on a cluster.