Various issues in SshComputerSetup

When looking at the code of `SshComputerSetup` I noticed a couple of (apparent) issues.

- When a config for a given computer already exists, the (potentially) new values are not written. What's worse, the user is not notified about this. In essence this means that user cannot change an existing config
- When unit testing this widget, we're (repeatadly) writing to an actual `.ssh/config` file! We need to monkeypatch this.
- When a private Ssh key already exists, the new value is written to a different file, but this is not correctly propagated to the calling function