Add a staging overlay for the key vault name

Currently we're just using the test vault, but in the future we want to be able to prescribe vault names for different environments via overlay.

deploy/overlays/staging/kustomization.yaml

@@ -7,6 +7,7 @@ patchesStrategicMerge:
   - replica_count.yml
   - ports.yml
   - envvars.yml
+  - vault_name.yml
 patchesJson6902:
   - target:
       group: extensions

deploy/overlays/staging/vault_name.yml

@@ -0,0 +1,12 @@
+apiVersion: extensions/v1beta1
+kind: Deployment
+metadata:
+  name: atst
+spec:
+  template:
+    spec:
+      volumes:
+        - name: nginx-secret
+          flexVolume:
+            options:
+              keyvaultname: "atat-vault-test"