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.

site.yml

@@ -18,6 +18,12 @@
 
 - hosts: docker_swarm_manager_node
   tasks:
+    - name: Remove stacks without roles
+      import_role:
+        role: docker-clean
+      tags:
+        - docker-clean
+
     - name: Install caddy reverse proxy
       import_role:
         role: caddy