cyclades: Fix type error
cyclades: Fix volume type column in volume-list

Display only the ID of the volume type, and not the object's
representation.
cyclades: Accept snf-vol-$id to snf-manage cmds

Make management commands accept as argument the volume's prefixed ID.
cyclades: Display user's email in volume commands
cyclades: Fix bug in snapshot status

Because of a typo, snf-dispatcher always set the snapshot's status to
'AVAILABLE'. This commit fixes this issue.
cyclades: Fix issues in snapshot commands
plankton: Delay creation of Pithos backend pool

Delay the creation of the Pithos backend pool until an object from the
pool is actually needed. Until now, the pool was created when the
'synnefo.plankton.backend' module was imported. This caused an issue
to 'snf-dispatcher' which performs a 'fork()' in order to become a
daemon and 'objpool' which checks that the pid of the process who
creates and uses the pool is the same.
cyclades: Add setting for maximum volume size

Add 'CYCLADES_VOLUME_MAX_SIZE' setting to denote the maximum allowed
size for a Cyclades volume.
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.