Add basic caddy swarm role

Added caddy as swarm container. Uses templated Caddyfile which can be
set up for testing purposes for now.
Depends on docker and docker-swarm, and will keep its stuff where
docker-swarm sets itself up.

roles/caddy/vars/main.yml

@@ -0,0 +1,5 @@
+---
+
+caddy_stack:
+  - name: caddy
+    compose: "{{ lookup('template', 'docker-stack.yml.j2') | from_yaml }}"