Automatic merge from submit-queue
Kubelet: Remove nsinit related code and bump up minimum docker apiversion
Docker has native exec support after 1.3.x. We never need this code now.
As for the apiversion, because Kubernetes supports 1.8.x - 1.10.x now, we should bump up the minimum docker apiversion.
@yujuhong I checked the [changes](https://github.com/docker/engine-api/blob/master/types/versions/v1p20/types.go), we are not relying on any of those changes. So #23506 should work with docker 1.8.x+
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