update to use GA toplogy labels for PVs

[skriss]

As of Kubernetes 1.17, the labels that capture a PV's region & zone have gone GA:

• https://kubernetes.io/blog/2019/12/09/kubernetes-1-17-release-announcement/
• https://kubernetes.io/docs/reference/kubernetes-api/labels-annotations-taints/#topologykubernetesioregion
• https://kubernetes.io/docs/reference/kubernetes-api/labels-annotations-taints/#topologykubernetesiozone

Velero currently uses the older beta version of the zone label in at least one place that I'm aware of:

• https://github.com/vmware-tanzu/velero/blob/master/pkg/backup/item_backupper.go#L389

We probably want to use the GA version of the label, if it exists, otherwise fall back to the beta version. At some point we'll be able to drop support entirely for the beta version of the label -- we can add a TODO to the code for this.

[cpanato]

@skriss can i take this to try out?

[skriss]

yes, absolutely! Let me know if you need any further input beyond what I put in the initial description. The main thing to think about here is making sure we remain backwards-compatible.