chore: update ssh-key format in tests to match real-world usage

[7flying]

This change allows us to reproduce RHBZ#2218231 (and to catch more related bugs in the future)

and it also fixes that same BZ.

Our FSIM for creating users with an associated ssh key did not take into account that ssh keys are strings that may have spaces, e.g. 'ssh-ed25519 long-string user@example.com', and used ' ' as the split character.

This sets ';' as the split character in the FSIM avoiding installing unusable ssh keys.

[7flying]

One minor nit is the general accepted for non real domains is example.com, I think email.com is a real domain so we may not want to use that.

oh, I did not know that, will update this accordingly, thanks :+1:

[7flying]

@nullr0ute sorry about the dismissed review, I ended up fixing the BZ also in this PR

[7flying]

Linking #507 for awareness