Managing kernel upgrades

[nlewo]

Currently, a kernel upgrade is not well supported since the old kernel is still running after the kernel upgrade applied by terraform apply.
So, when the kernel on a host is upgraded, the host should be rebooted or the new kernel should be loaded with kexec.

This is hard for Terraform to handle this since we don't want to reboot the machine on every the configuration change. Maybe, this should be realized by the nixos-rebuild script itself.