hetzner: Set robot server name to vm_id.

This should make it easier for identifying deployed machines in the
robot, but it's also useful for sanity checks to not accidentally
overwrite existing deployments.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
hetzner: Truncate Robot's server name to 50 chars.

We want to avoid exceptions as the Robot unly allows 50 characters at
most. Sadly, even shorter hostnames will be truncated and might clutter
up the server list a bit.

But I've already sent a feature request to the Robot team at Hetzner, so
we might have longer names soon.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
hetzner: Use separate admin user for each machine.

This uses the changes introduced in version 0.4 of the Python Hetzner
API, which allow creation of separate admin accounts that only have
privileges to manage one single machine.

And that's exactly what we need, because we don't want to have the real
robot password hanging around in our machine state. So if we export a
deployment and sending it to thers, they only have access to the
machines in the deployment.

Of course, this also means, that hetzner.robotUser and hetzner.robotPass
only have to be set on machine creation and are no longer needed
afterwards.

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