Fixes #5683 - Use updated netboot image for Ubuntu Precise

The old one spends 10+ minutes deduplicating Package lists.
https://bugs.launchpad.net/ubuntu/+source/net-retriever/+bug/1067934
@ohadlevy if this gets fixed we would also need lucid-updates and saucy-updates as they contain the same bug. I think adding a checkbox (false by default) on OperatingSystem for Debian/Ubuntu families that changes ```release``` to ```release-updates``` would solve the issue easily.