Fix the kolla_docker issue with docker 1.12

After docker 1.12, the RepoTags will be None rather than [] in the
image list API. This PS will handle the both case.

Change-Id: Ie2da44b44229c2f190550755b50876f607f9cc0c
Closes-Bug: #1608358
(cherry picked from commit ae34973d35)

ansible/library/kolla_docker.py

@@ -228,7 +228,10 @@ class DockerWorker(object):
     def check_image(self):
         find_image = ':'.join(self.parse_image())
         for image in self.dc.images():
-            for image_name in image['RepoTags']:
+            repo_tags = image.get('RepoTags')
+            if not repo_tags:
+                continue
+            for image_name in repo_tags:
                 if image_name == find_image:
                     return image