feat(docker): Add docker stack cleaning role

Runs before setting up any new stacks or pursue other modifications to
docker deployments.

Brings down any stack which is not currently defined in a role. This
makes the whole installation more idempotent since we take care to not
only bring _up_ any necessary docker containers, but also bring _down_
those that have become unnecessary.

roles/docker-clean/tasks/main.yml

@@ -0,0 +1,12 @@
+---
+- name: Get running docker stacks
+  community.docker.docker_stack_info:
+  register: running_stacks
+  become: true
+
+- name: Remove stacks without matching role
+  community.docker.docker_stack:
+    name: "{{ item.Name }}"
+    state: "absent"
+  loop: "{{ running_stacks.results | rejectattr('Name', 'in', role_names) }}"
+  become: true