Use constants for retry automatically provided kwargs

Instead of using strings use module level constants for
the automatically provided keyword arguments to the retry
revert/execution functions. 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: I739a0ad69819c3ca0d10c98966012cf9ef1b86bd
Merge "Use constants for retry automatically provided kwargs"