reinstall: scrape root password

[omertuc]

I think we probably are going to need to scope creep this tool slightly into supporting non-cloud flows; I've seen people doing that already with to-existing-root - basically the scenario here is:

• Install Fedora Server (or whatever), have a physical console
• Want to replace it w/bootc image

In this flow we'd need to support a root password, not ssh keys. One general purpose option here would be to support running systemd-firstboot. Or of course, we could just try to automatically scrape a root password in addition too.

I kind of like that latter one as an expedient option - in clouds we typically won't have a root password so it'd be a no-op.

Originally posted by @cgwalters in https://github.com/containers/bootc/pull/1092#discussion_r1946488246