Add a flag to support VM creation without external IP

[barp]

When working with internal IP in a shared VPC there might not be any reason to create an external IP to the VM. And since external IP have quotas you can't have too many builds using it.

We can't do it by default when using internal IP since access to external repositories will require Cloud NAT to be enabled and it has costs associated with it.

[pjh]

cc @isarkis and @agvdndor who have also been looking at more sophisticated network options for the builder.

[barp]

Hi can you take a look again?

[pjh]

LGTM, thanks!

FYI I don't get email notifications when you push a new commit or resolve a comment, please add a comment somewhere when you'd like a re-review.