Automatic merge from submit-queue

Fixed mounting with containerized kubelet

`NsenterMounter.IsLikelyNotMountPoint()` should return `ErrNotExist` when the
checked directory does not exists - the regular mounted does this and
some volume plugins depend on this behavior.

See for example: https://github.com/kubernetes/kubernetes/blob/master/pkg/volume/aws_ebs/aws_util.go#L72