Adding '/usr/bin/env bash'

This commit aims to add '/usr/bin/env bash' as a shebang line
to indicates scripts use bash shell for interpreting.

Signed-off-by: Nguyen Hai Truong <truongnh@vn.fujitsu.com>

hack/update-generated-kubelet-plugin-registration-dockerized.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright 2018 The Kubernetes Authors.
 #

hack/update-generated-kubelet-plugin-registration.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright 2018 The Kubernetes Authors.
 #

hack/verify-generated-kubelet-plugin-registration.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright 2018 The Kubernetes Authors.
 #