List the containers on the system.

def list_containers(active=True, defined=True,
                    as_object=False, config_path=None):
    """
        List the containers on the system.
    """

    if config_path:
        if not os.path.exists(config_path):
            return tuple()
        try:
            entries = _lxc.list_containers(active=active, defined=defined,
                                           config_path=config_path)
        except ValueError:
            return tuple()
    else:
        try:
            entries = _lxc.list_containers(active=active, defined=defined)
        except ValueError:
            return tuple()

    if as_object:
        return tuple([Container(name, config_path) for name in entries])
    else:
        return entries