Add a randomized delay to borg-backup timers

Add an hour of delay to spread out borg backup jobs from multiple
servers and other jobs which are scheduled to run at OnCalendar=daily.

roles/borg-client/templates/borg-backup.timer.j2

@@ -4,6 +4,7 @@ Description=Borg backup
 [Timer]
 OnCalendar=daily
 Persistent=true
+RandomizedDelaySec=1h
 
 [Install]
 WantedBy=timers.target