Ensure that the runtime mounts RO volumes read-only

Add a feature gate ReadOnlyAPIDataVolumes to a provide a way to
disable the new behavior in 1.10, but for 1.11, the new
behavior will become non-optional.

Also, update E2E tests for downwardAPI and projected volumes
to mount the volumes somewhere other than /etc.

cluster/addons/dashboard/dashboard-controller.yaml

@@ -56,8 +56,7 @@ spec:
           timeoutSeconds: 30
       volumes:
       - name: kubernetes-dashboard-certs
-        secret:
-          secretName: kubernetes-dashboard-certs
+        emptyDir: {}
       - name: tmp-volume
         emptyDir: {}
       serviceAccountName: kubernetes-dashboard