functional: simplify nspawn machine creation

Generating all SSH host keys every time we spin up a machine is slow and
unnecessary. We could consider moving to fixtures, but for now it's easier to
simply generate a single key.

systemd also introduced a hardware database service that was taking a 
relatively considerable amount of time, and should be unecessary in a 
systemd-nspawn container.

This brings the average machine bring-up to ~1s