prow.sh: also configure feature gates for kubelet

That this hasn't been done before is an oversight. Apparently it
hasn't been a problem because there never have been feature gates that
mattered?

prow.sh

@@ -568,6 +568,13 @@ kubeadmConfigPatches:
   nodeRegistration:
     kubeletExtraArgs:
       "feature-gates": "$gates"
+- |
+  apiVersion: kubelet.config.k8s.io/v1beta1
+  kind: KubeletConfiguration
+  metadata:
+    name: config
+  featureGates:
+$(list_gates "$gates")
 - |
   apiVersion: kubeproxy.config.k8s.io/v1alpha1
   kind: KubeProxyConfiguration