Set nova_api_insecure option according to the nova deployment

Since we can't depend on a nova proposal (this would create a circular
dependency), we just take the first node with a nova controller role
that we find.
Always talk to nova over the admin network

We use the internalURL endpoint for non-admin usage, but that's still
the admin network.
Merge pull request #133 from vuntz/nova-config

Correctly setup the nova-related options