(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
May not apply to this use-case specifically, but what if we want to change what we store here in a later release? Seems like there's no versioning for the payload at the moment, so looks like we'd have to do a flaky check. May want to have some common code for handling payload versioning. Could be in another PR though.