cluster: enable debug handlers on GCE master nodes

This is needed for testing metrics support via the secure port
of kube-scheduler and kube-controller-manager. To access that
port, port-forwarding is used.

cluster/gce/util.sh

@@ -1025,7 +1025,7 @@ EOF
 # cat the Kubelet config yaml for masters
 function print-master-kubelet-config {
   cat <<EOF
-enableDebuggingHandlers: false
+enableDebuggingHandlers: ${MASTER_KUBELET_ENABLE_DEBUGGING_HANDLERS:-false}
 hairpinMode: none
 staticPodPath: /etc/kubernetes/manifests
 authentication:
@@ -1049,7 +1049,7 @@ EOF
 # cat the Kubelet config yaml in common between linux nodes and windows nodes
 function print-common-node-kubelet-config {
   cat <<EOF
-enableDebuggingHandlers: true
+enableDebuggingHandlers: ${KUBELET_ENABLE_DEBUGGING_HANDLERS:-true}
 EOF
   if [[ "${HAIRPIN_MODE:-}" == "promiscuous-bridge" ]] || \
      [[ "${HAIRPIN_MODE:-}" == "hairpin-veth" ]] || \