Root home fallback for keep users

[eriktate]

This PR falls back to using the root file path as a user's home directory in the case that their expected home directory already exists. This should prevent situations where a user that gets deleted and reprovisioned with a different UID/GID combo is unable to be used as a login for the host.

changelog: Fixed an issue preventing connections when using a newly provisioned host user whose home directory already existed.

[github-actions]

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[rosstimothy]

Instead of modifying the existing users home directory, can we leave it as is, but instead of starting a session in /home/user, start it in / like we seem to already do in some cases: https://github.com/gravitational/teleport/blob/master/lib/srv/reexec.go#L1049-L1064?

[eriktate]

Instead of modifying the existing users home directory, can we leave it as is, but instead of starting a session in /home/user, start it in / like we seem to already do in some cases: https://github.com/gravitational/teleport/blob/master/lib/srv/reexec.go#L1049-L1064?

For the path where we potentially convert an existing user into a keep user I could definitely see leaving their home directory alone and I probably should've done that to begin with. I'm not sure why we'd want to create new, managed users with an inaccessible home directory though?

[rosstimothy]

I'm not sure why we'd want to create new, managed users with an inaccessible home directory though?

The end goal is to have the new managed user be created, access to the host to be permitted, and eventually after some manual intervention own the pre-existing home directory. As is stands now with this PR, there is no warning to users that their home directory is/was inaccessible by the newly provisioned user and that we decided to make their home directory / instead. This also now requires a human to manually edit the permissions on the existing home directory and change the user's home directory to get to the desired end state.

[eriktate]

Closing this in favor of #47524