GCE: Improve check for ConnectWithPublicIPs()

[Brendan Cox]

Our check for ConnectWithPublicIPs() is metadata.OnGCE(). That isn't sufficient. We also need to make sure the controller is inside the same GCE private network as the client is configured to use.

To fix this, we can use gceClient.detectCurrentVPC() and ensure that matches the configured VPC.

[Vilmos Nebehaj]

Fixed via #109