add monitoring for Ubuntu mirror being in sync

see RT #3793 for an issue in 2012 when this
was not updated and went unnoticed for a long time

Matanya added a script that i slightly fixed
in I4d6e47ed7776a

This commit replaces that script entirely with a much better version
that has no fetching stuff for remote urls and as such is more secure
This is a slightly modified version of the script found in:
https://wiki.ubuntu.com/Mirrors/Monitoring%20Scripts

moving that around to run via NRPE and adding
it to installserver module, Ubuntu mirror class

Change-Id: I6024b3b57906ba0b023a94ba1375be71174a6e72