Support url based installs for Ubuntu

* copy/modify code for RedHat url based installs to Ubuntu
* modify UbuntuGuest for required attributes
* read grub configs for needed kernel/initrd information
* workarounds for different layout on `mini.iso` vs. standard Ubuntu images

Valid `<url>` values for Ubuntu installs
* [Updated Precise amd64](http://archive.ubuntu.com/ubuntu/dists/precise-updates/main/installer-amd64/current/images/netboot/)
* [Original release Quantal i386](http://archive.ubuntu.com/ubuntu/dists/quantal/main/installer-i386/current/images/netboot/)