Only offer hostcertificates if they exist (#15849) (#15853)

Only offer hostcertificates if they exist (#15849) (#15853)

Backport #15849

A common bug report is the otherwise harmless sshd logging:

```
Could not load host certificate "/data/ssh/ssh_host_ed25519_cert": No such file or directory
```

This PR simply checks if these files exist before creation of sshd_config and if
they do not exist, doesn't add a reference to them.

Fix #14110 amongst others.

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: 6543 <6543@obermui.de>