Switch to docker pull

If you don't have docker-py installed, this fails. We may not always
have it so let's just use docker pull for this deprecated service.

Change-Id: I481780993e959709caf3da59e6ae3f0b62d2d6a5
Related-Bug: #1870557

deployment/deprecated/docker/docker-baremetal-ansible.yaml

@@ -191,8 +191,7 @@ outputs:
           block:
             - name: Pre-fetch all the containers
               become: true
-              docker_image:
-                name: "{{ container_image }}"
+              shell: "docker pull {{ container_image }}"
               loop_control:
                 loop_var: container_image
               loop: "{{ lookup('file', tripleo_role_name + '/docker_config.yaml', errors='ignore') | default('{}', True) | from_yaml | recursive_get_key_from_dict(key='image') | unique }}"