Allow build-stack to set release name

When trying to run `sudo /vagrant/bin/build-stack 2.0.0 /vagrant/bin/cedar.sh` it breaks because of the hardcoded suite in `debootstrap --variant=minbase trusty $MNT`. I updated `build-stack` to accept a third argument to specify the `debootstrap` suite. If left empty, it will just default to `trusty`.