(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
This seems a bit fragile to me, but I can't think of a cleaner way of doing it. So any suggestions on how to make it better would be great.