cyclades: Add option to disable snapshots API

Add 'CYCLADES_SNAPSHOTS_ENABLED' setting to denote whether Cyclades
will expose the '/snapshots' API URL of the 'volume' app.
In case this setting is set to False:
* we disable the '/snapshots' API URL
* return 501 if the 'source_snapshot_id' attribute is used in a request
  for a new volume
* Remove 'snapshot' from the supported sources of a new volume in the
  create server request

Note: Creation and handling of snapshots can still be performed via
'snf-manage' commands. The setting affects only the exposed API.
cyclades: Change setting's default value
webproject: Remove wrong entries from package manifest

Two entries were mistakenly copied from the cyclades manifest.
cyclades: Fix setting for unittests
docs: minor fixes
docs: minor fixes
cyclades: Fix not null constraint

Image's 'os' and 'osfamily' attributes cannot be null.
cyclades: Move snapshot registration to plankton

Move the code that registers a snapshot to Pithos (using the
'register_object_map' method of the Pithos backend) to plankton.