Return VNC port in show_headnode

There's a little bit of (documented) trickiness, in haas/model.py

This needs to be deployment-tested.
Return port 'None' instead of -1
Document return value for no_dry_run decorator
Use an XML parser to parse XML
In no_dry_run, return return-value of called function
Make do without check_output in python-2.6
Merge pull request #199 from gsilvis/vncport

Return VNC port in show_headnode