cyclades: Get Pithos objects by their UUID
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.
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