Further updates to update.py

1. Adjust how import reforming is done to use a function
   to reform the import line (after modification).
2. Clean the old copy of taskflow (if it exists) before
   placing a new copy in its place, this is very useful
   to avoid stale files.
3. Move some constants to be offical global constants.
4. Show exactly what modules are being used from the
   containing projects openstack.common.
5. Fix how the distributed flow was breaking imports due
   to the location/usage of logbook in that module no
   longer being functionally correct.
6. Add a header to each file copied/created that tells
   people not to modify them directly.

Change-Id: I6387722e092b93418bd63b722a95a8efcaef788d
Merge "Further updates to update.py"