Get images and snapshots by their UUID

Make Plankton to use the newly introduced `get_object_by_uuid` method of `PithosBackend` in order
to retrieve images and snapshots by their UUID. The `--public` and `--user` options are removed
from `image-show` and `snapshot-show` management commands, since they are not longer
required.

Also, this PR includes some small fixes for release-0.16.