Add support for legacy registries to pull_image().

Since we now have several exceptions to the assumption that the
result of the pull would be on the last status line returned by
docker-py's pull(), I've changed the function so that it looks
through the status lines and returns what if finds on it.
Despite the repeated `break`s, the code seems simpler and a little
more coherent like this. From what I've checked using
`https://github.com/jlafon/ansible-profile`, the execution time is
mostly the same.

cloud/docker/docker.py

@@ -1390,24 +1390,20 @@ class DockerManager(object):
                 self.module.fail_json(msg="failed to login to the remote registry, check your username/password.", error=repr(e))
         try:
             changes = list(self.client.pull(image, tag=tag, stream=True, **extra_params))
-            try:
-                last = changes[-1]
-                # seems Docker 1.8 puts an empty dict at the end of the
-                # stream; catch that and get the previous instead
-                # https://github.com/ansible/ansible-modules-core/issues/2043
-                if last.strip() == '{}':
-                    last = changes[-2]
-            except IndexError:
-                last = '{}'
-            status = json.loads(last).get('status', '')
+            pull_success = False
+            for change in changes:
+                status = json.loads(change).get('status', '')
                 if status.startswith('Status: Image is up to date for'):
                     # Image is already up to date. Don't increment the counter.
-                pass
+                    pull_success = True
+                    break
                 elif (status.startswith('Status: Downloaded newer image for') or
                         status.startswith('Download complete')):
                     # Image was updated. Increment the pull counter.
                     self.increment_counter('pulled')
-            else:
+                    pull_success = True
+                    break
+            if not pull_success:
                 # Unrecognized status string.
                 self.module.fail_json(msg="Unrecognized status from pull.", status=status, changes=changes)
         except Exception as e: