pithos: Extend backend

Provide an additional backend method 'get_object_by_uuid'
which gets the object's UUID as an input
and return metadata and permission information
about the object.
pithos: PEP8 fixes
pithos: Slight change in get_domain_objects

This method gets two keyword arguments: user and check_permissions.
If check_permissions is True returns only the object accessible to the user.
Otherwise list all the objects for the specific domain ignoring
permissions.
The latter case is only meaningful if user is not provided,
therefore in that case it raises AssertionError.
cyclades: Get Pithos objects by their UUID

Use the newly introduced 'get_object_by_uuid' method to get a Pithos
object by it's 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 option to bypass
Pithos permission check.

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