r/winbind: Add domain-join tag

Sometimes I need to configure a machine to be a domain member without
actually adding it to the domain.  Now I can by running
`ansible-playbook` with `--skip-tags domain-join`

roles/winbind/tasks/main.yml

@@ -42,3 +42,5 @@
     username: '{{ winbind_join_username }}'
     password: '{{ winbind_join_password }}'
     state: joined
+  tags:
+  - domain-join