This PR contains Kubelet changes to enable attach/detach controller control.
* It introduces a new "enable-controller-attach-detach" kubelet flag to
  enable control by controller. Default enabled.
* It removes all references "SafeToDetach" annoation from controller.
* It adds the new VolumesInUse field to the Node Status API object.
* It modifies the controller to use VolumesInUse instead of SafeToDetach
  annotation to gate detachment.
* There is a bug in node-problem-detector that causes VolumesInUse to
  get reset every 30 seconds. Issue https://github.com/kubernetes/node-problem-detector/issues/9
  opened to fix that.