GCE instance should use private VPC and private IP

There should be options for launching the GCE instance into a private VPC and/or assign it a private IP. The default behavior of assigning a public IP at image launch causes failure due to org policy.

Start by tracing how compute-daisy launches GCE instances and where network, VPC, and IP settings are configured. Determine how options should represent a private VPC and private IP, and verify the default launch path no longer requires a public IP under the stated organization policy. Done means the requested private networking options are available and the launch succeeds without a public IP.

Written by the indexing model from the issue text.