Add "dry run" option to cleanup policy

We've seen a few requests now from users who really want to use the new cleanup feature, but are afraid to run it because they're not entirely sure what it will delete.  These users are aware of the preview feature, but don't find it to be sufficient.  

The common request from the above has been to add a "dry run" mode to cleanup policies.  When run in this mode they would log what they would delete to the task log, but would not actually delete anything.