Add service-images-pull tag to tasks in the service-images-pull role

We sometimes have the requirement that images should explicitly not
be pulled. Using the service-images-pull tag, it is now possible to
skip the actual pull task by using --skip-tags.

Change-Id: Ia00a5ecbcb944c252cd9d0366d8cf1e7ff6327f7

ansible/roles/service-images-pull/tasks/main.yml

@@ -14,3 +14,5 @@
   with_dict: "{{ lookup('vars', (kolla_role_name | default(project_name)) + '_services') | select_services_enabled_and_mapped_to_host }}"
   loop_control:
     label: "{{ item.key }}"
+  tags:
+    - service-images-pull