Close netbox ARP records after 30min downtime

This adds a default navclean job every five minutes to close ARP
records of devices that have been down for more than 30 minutes.
Needs to be a separate job line, since it doesn't use the built-in
default of 6 month expiry interval.

python/nav/etc/cron.d/dbclean

@@ -1,2 +1,3 @@
 ## info: Clean database with navclean
 */5 * * * * navclean -f -q --netbox --websessions
+*/5 * * * * navclean -f -q --close-arp --interval '30 minutes'