Grab the correct ServiceNet IP

This closes #740.
I'm not super great at either understanding or emitting communication atm, so I'll just say a thing which may or may not answer the question - a user can definitely create a server without a private net address, which is fine so long as they don't care about adding the server to a cloud load balancer.

See http://docs.rackspace.com/servers/api/v2/cs-devguide/content/CreateServers.html, the networks section.  If no networks are provided, by default it's added to both.  If networks are provided, only the networks listed are added to the server.  The important part of that document is:

    If you want to attach to the private ServiceNet or public Internet networks, 
    you must specify them explicitly. The UUID for the private ServiceNet is 
    11111111-1111-1111-1111-111111111111. The UUID for the public 
    Internet is 00000000-0000-0000-0000-000000000000.

So if they JUST provided the public net, there may not be a private address.  However I believe @lekhajee mentioned that Nova will return a 400 if you are a certain type of customer.