Fix Requests breaking download progress bar

The move to the requests library (dbb242b) broke the progress bar during
downloading an image with --progress enabled, due to requests returning
a generator, which has no __len__ function. This patch adds an iterable
wrapper which provides the length, sourced from Content-Length headers.

Closes-Bug: #1384664

Change-Id: I48598a824396bc6e7cba69eb3ce32e7c8f30a18a

glanceclient/v2/images.py

@@ -115,10 +115,12 @@ class Controller(object):
         url = '/v2/images/%s/file' % image_id
         resp, body = self.http_client.get(url)
         checksum = resp.headers.get('content-md5', None)
+        content_length = int(resp.headers.get('content-length', 0))
+
         if do_checksum and checksum is not None:
-            return utils.integrity_iter(body, checksum)
-        else:
-            return body
+            body = utils.integrity_iter(body, checksum)
+
+        return utils.IterableWithLength(body, content_length)
 
     def upload(self, image_id, image_data, image_size=None):
         """