feat(restic): Add restic backup maintenance stack

Sets up regular backup maintenance for a restic (S3) backend, and
enables global variables for other roles to use for their individual
backup. Example found in nextcloud role.

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