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/defaults/main.yml

@@ -0,0 +1,14 @@
+---
+restic_version: latest
+
+# restic_repo: s3.eu-central-1.wasabisys.com/myrepo
+# restic_pass: <restic-pass>
+# restic_s3_key: <s3-key>
+# restic_s3_secret: <s3-secret>
+restic_timezone: "{{ server_timezone | default('US/Chicago') }}"
+
+restic_prune_cron: 0 0 4 * * *
+restic_forget_args: --prune --keep-last 14 --keep-daily 2 --keep-weekly 2
+
+restic_check_cron: 0 15 5 * * *
+restic_check_args: --read-data-subset=5%