Adding delay monitoring for munin

Hi drolbr,

I don't know if that is of interest but we use munin to monitor the overpass DB lag (in case something goes wrong)

This patch adds a munin config file to copy in /etc/munin/plugins 
Note however that it only works for people using the fetch_osc_and_apply.sh update script (even if with a little change it could work for the usual update script)