(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
Actually `acls` can be null:

```
System.out.println(gson.fromJson("", String.class)); // null
System.out.println(gson.fromJson("", new TypeToken<List<ACL>>(){ }.getType())); // null
System.out.println(gson.fromJson("[]", new TypeToken<List<ACL>>(){ }.getType())); // []
```