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.