Make Cyclades work with Archipelago v2 and Pithos

Recent commits have integrated Archipelago v2 mapfiles to Pithos. This
commit updates Cyclades to go along with the recent changes in Pithos.
Specifically, this commit makes the following changes:

* Use the 'mapfile' attribute of the Pithos objects, instead of the old 'hash'
  attribute. Also, this attribute needs no custom prefixing
  ('archip:' or 'pithos:').
* Do not store 'is_snapshot' and 'status' attributes as plankton
  properties. The 'is_snapshot' attribute is now provided from Pithos.
  Also, the 'status' attribute can be derived from the 'available'
  attribute.
* Remove code from 'snf-dispatcher' that used to update the 'status'
  plankton property when the OP_INSTANCE_SNAPSHOT Ganeti job completed.
  This functionality is now covered internally from Pithos.
cyclades: Rename 'backend_id' to 'pithosmap'

Rename the 'backend_id' attribute of images to 'pithosmap'.
cyclades: Update tests for images/snapshots
ci: Use unstable repo for snf-image
deploy: Remove svipc module dependency

Update Gunicorn startup conf file.
ci: Install blktap-utils from unstable
deploy: Update archipelago conf file