text: Title: [v1.9.0] Kubelet unable pull image
  Body: I run a dashboard which used my private docker registry with kube 1.9.0.
  First, I had create one secret  ,and I can  pull image with "docker pull" use the username and password which in dockercfg.
  ![image](https://user-images.githubusercontent.com/312469/34070814-5c3128b8-e2a7-11e7-9043-f00fcf99bada.png)
  But, the kubelet can't pull image.
  ![image](https://user-images.githubusercontent.com/312469/34070753-c490c87e-e2a6-11e7-8b94-a656a04a6758.png)
  ![image](https://user-images.githubusercontent.com/312469/34070761-ddef74f0-e2a6-11e7-9a75-1013e70093bb.png)
  This is the kubelet log :
  ![image](https://user-images.githubusercontent.com/312469/34070789-1ec9d556-e2a7-11e7-9f06-a6aaa8e4cc73.png)
  
  