Use the correct `target_host`, if a user is defined.
Before the change:
```console
$ clan machines install
$ Install jon to 49.13.232.197? [y/N]
```
After the change:
```console
$ clan machines install
$ Install jon to root@49.13.232.197? [y/N]
```
Because root is the configured user for `target_host`, this is the
correct behavior.