engine : Introduction of TaskHelper interface

This patch introduces the task helper interface and
removes methods from AsyncTaskManager and SPMAsyncTask
and moves them to CommandCoordinator

Change-Id: I7c5b3db8d679096aab5d791ba61084d54c26b9de
Bug-Url: https://bugzilla.redhat.com/1083769
Signed-off-by: Ravi Nori <rnori@redhat.com>