hetzner: Truncate server name at 100 chars.

The increase of the server name length was part of my wishlist to the
folks at Hetzner which they implemented in the meantime (before, the
maximum length was 50 chars which leaves room for the UUID but truncated
host names at 6 chars). So thanks to the Hetzner Robot team for
increasing that limit.

For us, this means that we can store the full vm_id in the Robot which
currently only makes it easier to distinguish individial NixOps hosts in
the Robot.

In the long term however, it could be useful reconstructing/recovering
deploments (see #2).

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