Return VNC port in show_headnode

There's a little bit of (documented) trickiness, in haas/model.py

This needs to be deployment-tested.
That's... scary. More importantly, it relies on undocumented implementation details of `no_dry_run`, which makes this a no-go. I may be open to just documenting this. We should think about where this might go if we hit things that aren't as serendipitous, though.