Fix license boilerplate/copyright in go files (#947)

This change fix license boilerplate and its copyright.
The updated year in copyright is based on the file creation date.
If older than 2021, added copyright is transfered to multus
authors from Intel corporation as the multus code was officially
transfered to Kubernetes Networking Plumbing Working Group on
March 11, 2021.

pkg/kubeletclient/kubeletclient.go

@@ -1,3 +1,18 @@
+// Copyright (c) 2019 Intel Corporation
+// Copyright (c) 2021 Multus 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 kubeletclient
 
 import (