roles: docker: Ensure service is started if docker is already installed

If the 'docker' package is already installed, then the handlers will not
run and the service will not be (re-)started. As such, lets make sure
that the service is started even if the packages are already installed.

roles/docker/tasks/main.yml

@@ -118,6 +118,12 @@
   notify: restart docker
   when: not (ansible_os_family in ["CoreOS", "Container Linux by CoreOS"] or is_atomic) and (docker_package_info.pkgs|length > 0)
 
+- name: ensure service is started if docker packages are already present
+  service:
+    name: docker
+    state: started
+  when: docker_task_result is not changed
+
 - name: flush handlers so we can wait for docker to come up
   meta: flush_handlers