Support for deleting images

If an image is in glance and associated with a repo yet was not touched by pulp
during the publish, it will be removed. This is tracked via image checksums.

There is a failsafe to bail out totally if any images are in pulp that should
have been published and weren't.

The finalize() method is a little overly verbose and might be refactored in the future.
You don't re-use ``glance_images`` after iterating over it on the next line, so I don't think you need to cast it to a list.