Update nginx-ingress, Prometheus, and Grafana addons

* Align `nginx-ingress` `--controller-class` with `IngressClass`
to provide a better example (e.g. if extended to multiple ingress
controllers)

addons/nginx-ingress/google-cloud/class.yaml

@@ -3,4 +3,4 @@ kind: IngressClass
 metadata:
   name: public
 spec:
-  controller: k8s.io/ingress-nginx
+  controller: k8s.io/public

addons/nginx-ingress/google-cloud/deployment.yaml

@@ -26,6 +26,7 @@ spec:
           image: k8s.gcr.io/ingress-nginx/controller:v1.1.1
           args:
             - /nginx-ingress-controller
+            - --controller-class=k8s.io/public
             - --ingress-class=public
           # use downward API
           env: