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 bit overwrought; I wanted to make it obvious as to
what's happening since it involves deleting stuff but there may be room for
improvement without sacrificing readability.