In mongodb_consistent_backup/Common/Config.py you're assigning the user to config.user, however, the DB class (and other places) attempt to access config.username.
The most probable fix would be to Config.py:
- parser.add_argument("-u", "--user", dest="user", help="MongoDB Authentication Username (for optional auth)", type=str)
+ parser.add_argument("-u", "--user", dest="username", help="MongoDB Authentication Username (for optional auth)", type=str)