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.
s/string/str