Add an install_container_networking variable (default true)

* When `true`, the chosen container `networking` provider is installed during cluster bootstrap
* Set `false` to self-manage the container networking provider. This allows flannel, Calico, or Cilium
to be managed via Terraform (like any other Kubernetes resources). Nodes will be NotReady until you
apply the self-managed container networking provider. This may become the default in future.

bare-metal/flatcar-linux/kubernetes/bootstrap.tf

@@ -5,7 +5,7 @@ module "bootstrap" {
   cluster_name                    = var.cluster_name
   api_servers                     = [var.k8s_domain_name]
   etcd_servers                    = var.controllers.*.domain
-  networking                      = var.networking
+  networking                      = var.install_container_networking ? var.networking : "none"
   network_mtu                     = var.network_mtu
   network_ip_autodetection_method = var.network_ip_autodetection_method
   pod_cidr                        = var.pod_cidr

bare-metal/flatcar-linux/kubernetes/variables.tf

@@ -91,6 +91,12 @@ variable "networking" {
   default     = "cilium"
 }
 
+variable "install_container_networking" {
+  type        = bool
+  description = "Install the chosen networking provider during cluster bootstrap (use false to self-manage)"
+  default     = true
+}
+
 variable "network_mtu" {
   type        = number
   description = "CNI interface MTU (applies to calico only)"