Increase GCP network deletion timeout from 4m to 6m

* Removing a Google Cloud cluster often required 2 runs of
`terraform apply` because network resource deletes timeout
after 4m. Increase the network deletion timeout to 6m to
ensure apply only needs to be run once to remove a cluster
* Fixes a GCP errata item https://github.com/poseidon/typhoon/wiki/Errata

google-cloud/container-linux/kubernetes/network.tf

@@ -2,6 +2,10 @@ resource "google_compute_network" "network" {
   name                    = var.cluster_name
   description             = "Network for the ${var.cluster_name} cluster"
   auto_create_subnetworks = true
+
+  timeouts {
+    delete = "6m"
+  }
 }
 
 resource "google_compute_firewall" "allow-ssh" {