builder: ubuntu: Allow virt-install --os-variant to be specified on the command line.

Current virt-builder in F20 does not know about --os-variant=ubuntutrusty
so to build that you can now do:

  ./ubuntu.sh 14.04 trusty ubuntusaucy
builder: Add Ubuntu 14.04 disk image.

Also remove the Ubuntu 13.10 image, since it's not an LTS image.