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.
In case the server's image is left blank, we use the source
(image/snapshot) of the first volume. Also, if an image is specified
both for the server and for the first volume, we check that the image is
the same.
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.