Add enable_aggregation option (defaults to false)

* Add an `enable_aggregation` variable to enable the kube-apiserver
aggregation layer for adding extension apiservers to clusters
* Aggregation is **disabled** by default. Typhoon recommends you not
enable aggregation. Consider whether less invasive ways to achieve your
goals are possible and whether those goals are well-founded
* Enabling aggregation and extension apiservers increases the attack
surface of a cluster and makes extensions a part of the control plane.
Admins must scrutinize and trust any extension apiserver used.
* Passing a v1.14 CNCF conformance test requires aggregation be enabled.
Having an option for aggregation keeps compliance, but retains the
stricter security posture on default clusters

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

@@ -153,3 +153,9 @@ variable "enable_reporting" {
   description = "Enable usage or analytics reporting to upstreams (Calico)"
   default     = "false"
 }
+
+variable "enable_aggregation" {
+  description = "Enable the Kubernetes Aggregation Layer (defaults to false)"
+  type        = "string"
+  default     = "false"
+}