Kubelet flags take precedence

This changes the Kubelet configuration flag precedence order so that
flags take precedence over config from files/ConfigMaps.

See #56171 for rationale.

Note: Feature gates accumulate with the following
precedence (greater number overrides lesser number):
1. file-based config
2. dynamic cofig
3. flag-based config

Kubernetes-commit: 42589266407e32fdf716d1fef689f0aee7142cd9

pkg/util/flag/BUILD

@@ -32,6 +32,7 @@ go_library(
         "map_string_bool.go",
         "map_string_string.go",
         "namedcertkey_flag.go",
+        "noop.go",
         "omitempty.go",
         "string_flag.go",
         "tristate.go",

pkg/util/flag/noop.go

@@ -0,0 +1,41 @@
+/*
+Copyright 2018 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package flag
+
+import (
+	goflag "flag"
+	"github.com/spf13/pflag"
+)
+
+// NoOp implements goflag.Value and plfag.Value,
+// but has a noop Set implementation
+type NoOp struct{}
+
+var _ goflag.Value = NoOp{}
+var _ pflag.Value = NoOp{}
+
+func (NoOp) String() string {
+	return ""
+}
+
+func (NoOp) Set(val string) error {
+	return nil
+}
+
+func (NoOp) Type() string {
+	return "NoOp"
+}