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

infra/gcp/clusters/OWNERS

@@ -0,0 +1,13 @@
+# See the OWNERS docs at https://go.k8s.io/owners
+
+approvers:
+- ameukam
+- spiffxp
+reviewers:
+- ameukam
+- dims
+- spiffxp
+- thockin
+
+labels:
+- area/terraform