cyclades: Bug fixes in volume related commands

* Add a check to both name length and disk template length in
snf-manage volume-type-create.
* Add a check to name length of snapshots.
* Add a generic try/except clause to handle Pithos responses.
* Add --user/--public options in snapshot-show/image-show.
* Add a boolean "snapshot" field in image-list.
* Remove unused user agrument from backend.get_snapshot().
* Various typo fixes.