Store the updated configuration in a file.

    By default uses the filename "config.json", but that can be changed by
    setting the config_filename config entry.

def save_config(_config, _log, config_filename='config.json'):
    """
    Store the updated configuration in a file.

    By default uses the filename "config.json", but that can be changed by
    setting the config_filename config entry.
    """
    if 'config_filename' in _config:
        del _config['config_filename']
    _log.info('Saving config to "{}"'.format(config_filename))
    save_config_file(flatten(_config), config_filename)