cyclades: Add index to volume.deleted column
cyclades: Forbid actions to deleted resources

Do not allow any "write" action to deleted (marked as deleted=True)
servers, networks and volumes. Only reading information about the
deleted resources is allowed.
cyclades: Store the index when creating a volume

Store the volume's index in the DB when creating a new volume.
cyclades: Update volumes state if create VM fails

Update the state of server's volumes to 'ERROR', if Cyclades fails to
issue the instance creation job to the Ganeti backend.
cyclades: Get default volume type from server

Fix the 'volume-create' management command in order to use the server's
volume type, if the user did not specified it using the '--volume-type'
option.
cyclades: Do not allow cloning a volume

Update 'volume' app to not allow cloning a volume, because it is
currently not supported by Archipelago.
cyclades: Accept an empty image when using volumes

Accept an empty 'imageRef' attribute when creating a server with
volumes. The attribute must always be present even with an empty value.
Also, if not empty, the image must exist, since it is stored in the
server's DB entry.
cyclades: Delete snapshots by their UUID

Use the 'delete_by_uuid' method that was introduced in PithosBackend in
order to safely delete a snapshot by it's uuid.