infra/clusters: add OWNERS
infra/clusters: add OWNERS

We have leaf owners for every cluster, so why this? Two reasons:
- apply `area/terraform` to all PRs here
- ameukam and myself have more TF knowledge than the other k8s-infra
  leads, I would like to signify that we should be consulted before
  smashing that /approve button

kubernetes/k8s.io,kubernetes/k8s.io,kubernetes/k8s.io,kubernetes/k8s.io