Add a script to bootstrap a minimal ubuntu chroot for easier testing of braid

Instead of setting up virtual machines etc I find it easier to run Ubuntu in a chroot environment using systemd-nspawn.

This script uses debootstrap to create a basic chroot and then does just enough configuration to allow the braid base.bootstrap and braid trac.install scripts to be run against it.

It starts openssh on port 0.0.0.0:22 so it won't work if there's already and ssh server listening on the host machine.