Remove docker_image sections (unused)

We don't use docker_image for anything. It is a remant of the
pre-composable docker templates and we can now remove it.

This patch removes references to the 'docker_image' section
from docker/post.yaml and all of the docker/services* templates.

Change-Id: I208c1ef1550ab39ab0ee47ab282f9b1937379810

docker/services/nova-placement.yaml

@@ -49,10 +49,6 @@ outputs:
         get_attr: [NovaPlacementBase, role_data, step_config]
       service_config_settings: {get_attr: [NovaPlacementBase, role_data, service_config_settings]}
       # BEGIN DOCKER SETTINGS
-      docker_image: &nova_placement_image
-        list_join:
-          - '/'
-          - [ {get_param: DockerNamespace}, {get_param: DockerNovaPlacementImage} ]
       puppet_config:
         config_volume: nova_placement
         puppet_tags: nova_config
@@ -96,7 +92,10 @@ outputs:
         step_3:
           nova_placement:
             start_order: 1
-            image: *nova_placement_image
+            image:
+              list_join:
+                - '/'
+                - [ {get_param: DockerNamespace}, {get_param: DockerNovaPlacementImage} ]
             net: host
             user: root
             restart: always