write config data to file.

           Parameters
           ==========
           config_data: the Configuration object to save a username and
                        password, if defined
           indent: number of tab indentations to use when writing json

def _write_config_file(self, config_data, indent=2):
        """write config data to file.

           Parameters
           ==========
           config_data: the Configuration object to save a username and
                        password, if defined
           indent: number of tab indentations to use when writing json
        """
        with open(self.config, 'w') as f:
            json.dump(config_data, f, indent=indent)