Avoid manual reboot in unattended mode

[jreidinger]

In general if user use unattended mode, then now it needs to manually click on reboot. That is not expected behavior and ideally in unattended mode it should reboot automatically after successful installation.

[imobachgs]

Although there is an unattended mode from users POV, strictly speaking, there is nothing like that. At first sight, we could:

• Implement a sort of installer options that would allow to instruct Agama to reboot at the end. In the future, it could be use to configure other aspects of Agama.
• Add a call to systemctl reboot in our auto-installation service (simple). We could even create an agama reboot command that could be used remotely.