[RFE] nm-cloud-setup: add support for Google Cloud load-balancing routes

This is a feature request to teach nm-cloud-setup how to setup and reconcile custom routing rules for load-balanced machines on Google Cloud Platform (GCP).

On GCP, machines sitting behind a load-balancer (LB) need additional routing setup in order to properly participate in traffic load-balancing. The architecture and rationale is explained in https://cloud.google.com/load-balancing/docs/internal#how_ilb_works, and the forwarded-ips metadata field is described at https://cloud.google.com/compute/docs/storing-retrieving-metadata.

There are currently several hackish implementations of this route-setup logic:

• the old GCP agent (source)
• the new GCP agent (source)
• a custom setup script in OpenShift (source)

It would be better to have NetworkManager being directly aware of this setup/reconciliation logic. According to @thaller nm-cloud-setup is exactly intended for this usecase.

Ref: https://github.com/coreos/fedora-coreos-tracker/issues/67#issuecomment-458792347