Holland xtrabackup util.py should accept stream=none

[MikeG]

util.py 83:
 if stream_method is not None:
        LOG.warning("Skipping --prepare/--apply-logs since backup is streamed")
        return

util.py 126:
 if stream in ('no', '0', 'false'):
        return None
    raise BackupError("Invalid xtrabackup stream method '%s'" % stream)