(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
So this will change again when we want to support service ACL? Should it be just a ACL, which for local service case, the ACL contains the local appId only?