'Get the image string, either from the argument, or from the
configured interface default if ``image`` is ``None``. Optionally
resolve the image to its digest if ``resolve_image_digest`` is set in
the interface configuration.
If the image has not been pulled, the repo digest cannot be determined
and the image name will be returned.
:type   image:  str or None
:param  image:  image id or name
:returns:       image name or image repo digest'

def _get_image(self, image):

    if (not image):
        image = self._conf.image
    assert (image is not None), 'No    image    supplied    as    parameter    and    no    image    set    as    default    in    config,    cannot    create    service'
    if self._conf.resolve_image_digest:
        image = self.image_repodigest(image)
    return image
