Adds support for webhook token auth. (#3939)

Webhook token auth:
https://kubernetes.io/docs/reference/access-authn-authz/authentication/#webhook-token-authentication

Fixes #3063.

roles/kubernetes/master/templates/webhook-token-auth-config.yaml.j2

@@ -0,0 +1,17 @@
+# clusters refers to the remote service.
+clusters:
+- name: webhook-token-auth-cluster
+  cluster:
+    server: {{ kube_webhook_token_auth_url }}
+
+# users refers to the API server's webhook configuration.
+users:
+- name: webhook-token-auth-user
+
+# kubeconfig files require a context. Provide one for the API server.
+current-context: webhook-token-auth
+contexts:
+- context:
+    cluster: webhook-token-auth-cluster
+    user: webhook-token-auth-user
+  name: webhook-token-auth