Check whether GCEFirewall needs to be updated

[nuwang]

The current GCE firewall implementation is pretty complex.
As I recall, it uses network tags and simulated rules, including a dummy rule because GCE only had firewall rules, no firewall object. However, there now appears to be a GCE firewall resource: https://cloud.google.com/compute/docs/reference/rest/v1/firewalls

Assuming this refers to the same concept, we could significantly simplify the current implementation, including removal of the FirewallDelegate.

[nuwang]

In GCP, the firewall rule is referred to as a firewall.