SSH host keys are regenerated each time a VM does a stop/start

### Description

This is a (hopefully) unintended side effect of #273. Because the `instance-id` changes, `cloud-init` treats it like it's the first boot and does all the first boot things, like deleting the existing SSH host keys and regenerating new ones. I was able to work around this in an existing VM by creating a file in `/etc/cloud/cloud.cfg.d` with `ssh_deletekeys: false`, but this is not an ideal solution.