add stable os label kubernetes.io/os

[mingqishao]

Why this changes?

1. We are going to support Windows container in ACI VK, and allow customer to use node label to select windows virtual node. Giving the Kubernetes document, the node label "kubernetes.io/os" should be used for selecting window node.
2. Currently the os label in VK is beta.kubernetes.io/os. Kubernetes has started to use new label "kubernetes.io/os" and deprecated the beta one from v1.18

What is the changes

Add a new OS labels to VK.