engine : CommandBase should not set end status for commands with callback

When mixing async tasks and async commands if
a callback is provided by the command the CommandBase
should not set the status of the command. The
command should override endSuccessfully and
endWithFailure to set the status of the async
tasks status to succeeded or failed.

Change-Id: I84c05917003376d2f62d3da7acecd889f8cb79e0
Bug-Url: https://bugzilla.redhat.com/1160872
Signed-off-by: Ravi Nori <rnori@redhat.com>