Use constants for revert automatically provided kwargs

Instead of using strings use module level constants for
the automatically provided keyword arguments to the tasks
revert function. This makes it easier for users of taskflow
to associate these constants with the actual keywords, without
having to resort to using raw strings directly.

Change-Id: I12726812615052d8405071d46272ef2b8286cfe2
Merge "Use constants for revert automatically provided kwargs"