Refactor mkimage-debootstrap.sh to be much more full-featured

This includes the following:
- options to control certain parameters
- an option specifically for creating tarballs directly, esp. for dockerbrew
- the addition of the updates and security repositories for Debian images
- the addition of the universe, updates, and security repositories for Ubuntu images
- more correct tagging of Debian images
- tagging of Ubuntu image versions (12.04, 12.10, etc) and latest tag for LTS