Previously, the openstack.image.image_download method would place the
contents of a remote image into a Python variable. With this change,
the download_image method can optionally return the
requests.Response object returned by session.get(), which permits
the caller to download the image in chunks using the iter_content
method. This can prevent performance issues when dealing with
large images.
Change-Id: Ie62ebcc895ca893321a10def18ac5d74c7c843b9
5b2df7e724