Fix the error when pull keystone image

When pull keystone image, it will failed, msg is "image' is undefined",
this patch fix it.

Change-Id: I510030cc20a44410a847ab0a6ef36effae716975

ansible/roles/keystone/tasks/pull.yml

@@ -3,8 +3,8 @@
   kolla_docker:
     action: "pull_image"
     common_options: "{{ docker_common_options }}"
-    image: "{{ item.image }}"
+    image: "{{ item.value.image }}"
   when:
-    - inventory_hostname in groups[image.group]
-    - image.enabled | bool
+    - inventory_hostname in groups[item.value.group]
+    - item.value.enabled | bool
   with_dict: "{{ keystone_services }}"