docs: def ensure_image(self):
          """
          Ensure that config.image_url has been downloaded and unpacked.
          """
          image_folder = self.get_image_folder()
          if os.path.exists(image_folder):
              print(
                  'OS image directory {} exists...not overwriting' .format(
                      image_folder))
              return
  
          ensure_image(
              self.config.image_name,
              self.config.image_url,
              IMAGES_ROOT,
              getattr(self.config, 'image_md5', None),
              self.get_image_folder()
          )