(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
You're entirely correct. I couldn't quite figure out the best way to do this - suggestions? 

Similar logic is scattered in 3 places right now. So a way to consolidate would also be good.