cyclades: Catch corner case in reconciliation

Include 'ERROR' in the states for which we check if the NIC exists in
the Ganeti instance.
cyclades: Add snapshot property in lowercase

Snapshot properties are stored in lowercase in Pithos DB. It is the
image/plaknton API that converts them to uppercase.
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.
ci: Update flavor option for burnin

Do not use only file disk templates, since an archipelago disk template
is required for snapshot tests.