Add support for volumes & snapshots

This patch set extends Cyclades and Pithos to support Volumes and Snapshots.
Each virtual server can have multiple virtual disks (of the same disk
template), which can be dynamically created or removed. Also, in case of
Archipelago volumes, the user can create snapshots of an instance's volumes
which are stored as files on Pithos. Handling of volumes and snapshots is
performed via the OpenStack Cinder API, which is exposed by the Cyclades
'volume' service.