Return the YAML configuration file used to store upgrade information.

def _get_install_config():
    """Return the YAML configuration file used to store upgrade information.
    """
    try:
        data_dir = _get_data_dir()
    except ValueError:
        return None
    config_dir = utils.safe_makedir(os.path.join(data_dir, "config"))
    return os.path.join(config_dir, "install-params.yaml")