Add experimental Cilium CNI provider

* Accept experimental CNI `networking` mode "cilium"
* Run Cilium v1.8.0-rc4 with overlay vxlan tunnels and a
minimal set of features. We're interested in:
  * IPAM: Divide pod_cidr into /24 subnets per node
  * CNI networking pod-to-pod, pod-to-external
  * BPF masquerade
  * NetworkPolicy as defined by Kubernetes (no L7 Policy)
* Continue using kube-proxy with Cilium probe mode
* Firewall changes:
  * Require UDP 8472 for vxlan (Linux kernel default) between nodes
  * Optional ICMP echo(8) between nodes for host reachability
    (health)
  * Optional TCP 4240 between nodes for endpoint reachability (health)

Known Issues:

* Containers with `hostPort` don't listen on all host addresses,
these workloads must use `hostNetwork` for now
cilium/cilium#12116
* Erroneous warning on Fedora CoreOS
cilium/cilium#10256

Note: This is experimental. It is not listed in docs and may be
changed or removed without a deprecation notice

Related:

* poseidon/terraform-render-bootstrap#192
* cilium/cilium#12217

bootstrap.tf

@@ -1,6 +1,6 @@
 # Kubernetes assets (kubeconfig, manifests)
 module "bootstrap" {
-  source = "git::https://github.com/poseidon/terraform-render-bootstrap.git?ref=e75697ce35d7773705f0b9b28ce1ffbe99f9493c"
+  source = "git::https://github.com/poseidon/terraform-render-bootstrap.git?ref=af36c539360696f5ca6cf5b06bb729477a003602"
 
   cluster_name          = var.cluster_name
   api_servers           = [format("%s.%s", var.cluster_name, var.dns_zone)]

network.tf

@@ -112,6 +112,32 @@ resource "google_compute_firewall" "internal-vxlan" {
   target_tags = ["${var.cluster_name}-controller", "${var.cluster_name}-worker"]
 }
 
+# Cilium VXLAN
+resource "google_compute_firewall" "internal-linux-vxlan" {
+  count = var.networking == "cilium" ? 1 : 0
+
+  name    = "${var.cluster_name}-linux-vxlan"
+  network = google_compute_network.network.name
+
+  allow {
+    protocol = "udp"
+    ports    = [8472]
+  }
+
+  # Cilium health
+  allow {
+    protocol = "icmp"
+  }
+
+  allow {
+    protocol = "tcp"
+    ports    = [4240]
+  }
+
+  source_tags = ["${var.cluster_name}-controller", "${var.cluster_name}-worker"]
+  target_tags = ["${var.cluster_name}-controller", "${var.cluster_name}-worker"]
+}
+
 # Allow Prometheus to scrape node-exporter daemonset
 resource "google_compute_firewall" "internal-node-exporter" {
   name    = "${var.cluster_name}-internal-node-exporter"