Documentation/tls: add aggregated CA

This commit ports the documentation that was originally part of
coreos/tectonic-installer#2850.

Documentation/tls/tls-kube.md

@@ -30,11 +30,14 @@ Configure the user provided certificate paths in your platform, i.e. `platforms/
 module "kube_certs" {
   source = "../../modules/tls/kube/user-provided"
 
-  ca_cert_pem_path        = "/path/to/ca.crt"
-  kubelet_cert_pem_path   = "/path/to/kubelet.crt"
-  kubelet_key_pem_path    = "/path/to/kubelet.key"
-  apiserver_cert_pem_path = "/path/to/apiserver.crt"
-  apiserver_key_pem_path  = "/path/to/apiserver.key"
+  aggregator_ca_cert_pem_path   = "/path/to/aggregator-ca.crt"
+  ca_cert_pem_path              = "/path/to/ca.crt"
+  kubelet_cert_pem_path         = "/path/to/kubelet.crt"
+  kubelet_key_pem_path          = "/path/to/kubelet.key"
+  apiserver_cert_pem_path       = "/path/to/apiserver.crt"
+  apiserver_key_pem_path        = "/path/to/apiserver.key"
+  apiserver_proxy_cert_pem_path = "/path/to/apiserver-proxy.crt"
+  apiserver_proxy_key_pem_path  = "/path/to/apiserver-proxy.key"
 }
 ```