explicit kubelet config key in Node.Spec.ConfigSource.ConfigMap

This makes the Kubelet config key in the ConfigMap an explicit part of
the API, so we can stop using magic key names.

As part of this change, we are retiring ConfigMapRef for ConfigMap.

test/integration/apiserver/print_test.go

@@ -50,7 +50,6 @@ var kindWhiteList = sets.NewString(
 	"ExportOptions",
 	"GetOptions",
 	"ListOptions",
-	"NodeConfigSource",
 	"NodeProxyOptions",
 	"PodAttachOptions",
 	"PodExecOptions",