docs: def vars_args(parser):
      """Add various command line options for external vars"""
      parser.add_argument('--extra-vars',
                          dest='extra_vars',
                          help='Extra template variables',
                          default=[],
                          type=str,
                          action='append')
      parser.add_argument('--extra-vars-file',
                          dest='extra_vars_file',
                          help='YAML files full of variables',
                          default=[],
                          type=str,
                          action='append')