server: Add logging for observed_reboot().

This is one of the main reasons we want to have logging, because a
"hetznerctl rescue <server>" usually prints nothing until we either have
a shell or we timed out waiting for the machine to come up.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
server: Use logger named after server number.

This should make it easier to see which server the log message actually
is for.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
hetznerctl: Enable logging for INFO messages.

This should at least make observed_* actions a bit more visible and we
can easily switch between logging levels.

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