Use http.debian.net as default mirror

As stated at http://http.debian.net/ - the advantages of
http.debian.net over cdn.debian.net are for example:

* Mirrors whose copy is not in /debian (e.g. /pub/debian)
* Immediate effect on database changes (no DNS propagation lag)
* Lower chance of inconsistency
* Faster downloads (when APT is used, via parallel downloads)

Acked-by: Alexander Wirt <formorer@grml.org>
Acked-by: Evgeni Golov <evgeni@grml.org>