Allow horizon role to pull images

Change-Id: Iea91c39733d56a9291287a78a44756d8d38ca71d
Partially-Implements: blueprint pre-pull-images

ansible/roles/horizon/tasks/deploy.yml

+---
+- include: config.yml
+
+- include: start.yml

ansible/roles/horizon/tasks/main.yml

 ---
-- include: config.yml
-
-- include: start.yml
+- include: "{{ action }}.yml"

ansible/roles/horizon/tasks/pull.yml

+---
+- name: Pulling horizon image
+  kolla_docker:
+    action: "pull_image"
+    common_options: "{{ docker_common_options }}"
+    image: "{{ horizon_image_full }}"
+  when: inventory_hostname in groups['horizon']