Force hostname, regardless of DHCP or reverse DNS

Took me a while to figure this out, but my hostname got set to `10`, probably due to a DHCP server on the network giving `10.x.x.x`. I fixed this by setting `netcfg/hostname` as per the [Wheezy example preseed.cfg](http://www.debian.org/releases/wheezy/example-preseed.txt).