GCE: Option to skip public IP address association and use private IP address

[4e6]

This feature was implemented for EC2 in #221

Often one wants to block SSH on the public IP address of the EC2 instance and only
expose SSH on the internal IP. It would be great if NixOps could fallback to, or prefer, the private IP
of the target when deploying.

I briefly looked into GCE backend implementation, it seems there is already some support for networking with private IPs.

It lacks (compared to ec2 implementation):

• deployment.gce.associatePublicIpAddress option to skip public IP address association
• deployment.gce.usePrivateIpAddress seems unnecessary, because of GCE: Use private ip if possible #443 (GCE: Use private ip if possible). Additionally we should make address_to GCE function return private_ipv4 if public_ipv4 == None