Notifies double for each prune/check run which may need to be fixed.
Also custom notification contents cannot currently be passed.
Lastly, we should put identifying information into the notification body
(such as the hostname/container name for which the notification is
relevant).
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.