Adds TaskStub class

Partial Task patch, Id I4fbadc9a97e3147128c7c733384c7bb50918806f
removed the result and message vars of the Tasks class. However,
the succeed and fail methods were using those to update them
respectively. This patch introduces a new TaskStub class which
enables the PartialTask functionality without looking result
and message attrs of the Task class.

Fixes bug 1284975

Change-Id: I0b406e4662cfd3cb496b71be77ad10a90c178baa
Merge "Adds TaskStub class"