restic: Add stack

Adapted from cloudserve-infrastructure, implements a backup stack using
restic. The actual backups have to be implemented by individual other
roles but this sets up initialization, pruning and checking of a repository.

roles/restic/vars/main.yml

@@ -0,0 +1,8 @@
+---
+stack_name: restic
+
+stack_image: "mazzolino/restic"
+
+stack_compose: "{{ lookup('template', 'docker-stack.yml.j2') | from_yaml }}"
+
+backup_enable: true