Provide a way to re-install a system

[stgraber]

With current IncusOS, booting the installer on an existing system fails with an error even if the install seed instructs it to wipe the target drive. This is usually a good safety net to prevent accidental booting into the install media on an existing system causing data loss but it also makes it very tricky to reinstall systems sometimes.

We may want a way to override this check by having the user provide something unique from the existing installation to trigger the reinstall. I'm basically thinking of:

• Boot the installer
• Installer detects an existing IncusOS install
• Installer fails with the error saying there's an existing installation and to force a reinstall, to set force_reinstall: FOOBAR in the install seed
• FOOBAR would be a short string of character that's based on the content of the system's current root partition, so it would only be valid for a particular installation, making force_reinstall only work exactly once