One notify in tacker is not correct

There is a notify "Restart tacker containers" in `roles/tacker/tasks/config.yml`:

```

- name: Copying over existing policy.json
  template:
    src: "{{ node_custom_config }}/tacker/policy.json"
    dest: "{{ node_config_directory }}/{{ item.key }}/policy.json"
  register: tacker_policy_jsons
  when:
    - inventory_hostname in groups[item.value.group]
    - item.value.enabled | bool
    - tacker_policy.stat.exists
  with_dict: "{{ tacker_services }}"
  notify:
    - Restart tacker containers

```

But no handler named "Restart tacker containers" is defined, so when user use a custom policy file for tacker, the deployment will be failed.