Allow memcached role to pull images

Change-Id: I860af348b126979fa756f89c6fac769c750be85a
Partially-Implements: blueprint pre-pull-images

ansible/roles/memcached/tasks/deploy.yml

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

ansible/roles/memcached/tasks/main.yml

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

ansible/roles/memcached/tasks/pull.yml

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