hetzner: Alias main_ipv4 to public_ipv4.

As we really can't decide whether the IP address is private or public
_and_ the IP is mandatory for identifying the server, we just alias the
address here in order to let it show on "nixops info".

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
hetzner: Show as much as possible on check.

We didn't properly set res.exists and res.is_up, so it didn't show up on
"nixops check". This should be fixed new.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>