Add a way to request the guest to be rebooted after artifact installation

If the user wants to test an artifact that requires a reboot to become testable (kernel, systemd, ...), it is necessary to reboot the machine after artifact installation. That doesn't seem possible with the current TF API (short of hacking the reboot into the test plan, but the tests should be independent of guest setup).

Please add a flag into the API to request a reboot after all artifacts (repo/COPR/koji/...) are installed.