cyclades: Update snapshot based on Ganeti job

Make Cyclades update the state of a snapshot in Pithos DB based on
the result of the corresponding Ganeti job. This is required to be
performed by Cyclades, because Pithos has no way to know whether an
unavailable snapshot will ever by completed or not (e.g. the Ganeti
job failed or it still has not completed).

Refs grnet/synnefo#231
cyclades: Reconciliation for stale snapshots

Extend 'reconcile-servers' command to perform reconciliation for
snapshots, which can occur if the Ganeti job failed but the
corresponding message from eventd has never been processed. In order to
make reconciliation possible, Cyclades store a special metadata on the
snapshot that contains the Ganeti ID and the job ID that will create the
snapshot.

Closes grnet/synnefo#231