Use the newly introduced 'get_object_by_uuid' method to get a Pithos
object by its UUID, instead of mapping a UUID to a (account, container,
name) tuple.

Also, remove the '--user' and '--public' options from 'image-show' and
'snapshot-show' management commands. Instead, use the
'check_permissions' option to bypass Pithos permission check.

Finally, fix plankton tests to mock the 'get_object_by_uuid' method.