Automatic merge from submit-queue

Kubeadm - Make code OS-agnostic

**What this PR does / why we need it**:
Kubernetes just got Windows Containers alpha support, opening the opportunity for kubeadm to also be used to setup, at least, Windows Server 2016 or newer worker nodes. With that in mind, we should take the effort of writing OS-agnostic code whenever possible, e.g. when dealing with the filesystem.

**Which issue this PR fixes** 
https://github.com/kubernetes/kubeadm/issues/108

**Special notes for your reviewer**:

**Release note**: