Add default variables to docker-swarm role

Variable `docker_stack_files_dir` can be used if other stack deployments
need to copy over configuration files or other extra files. Defaults to
`/stacks` but can be set to anything and other roles should take care to
respect this and put their files relative to it.

Variable `docker_swarm_public_network_name` can be used to set the name
of the network to be created that faces outwards (ingress network) and
to which all stacks will be connecting.

roles/docker-swarm/tasks/main.yml

@@ -19,7 +19,14 @@
 
 - name: Ensure public network exists
   community.docker.docker_network:
-    name: public
+    name: "{{ docker_swarm_public_network_name }}"
     driver: overlay
     state: present
   become: true
+
+- name: Ensure stack files directory exists
+  ansible.builtin.file:
+    path: "{{ docker_stack_files_dir }}"
+    state: directory
+    mode: '0755'
+  become: true