Is the key mandatory?
If so you might would want to add some sanity checks if there is one.

E.g.:
```python
 # Both swift credentials need to be set - or none
 if bool(CONF.backup_swift_user) != bool(CONF.backup_swift_key):
        err = _('Configuration options "backup_swift_user" and "backup_swift_key" booth needs to be set, or none of them.')
        raise ValueError(unicode(err))

 if CONF.backup_swift_user and CONF.backup_swift_key:
     ....
```