Disable Grafana login form, admin user can't be disabled

* Example manifests aim to provide a read-only dashboard visible
to any users with network access (i.e. kubectl port-forward, LAN)
* Problem: Grafana always has an admin user, even with the user
management system disabled
* Disable the login form to prevent admin login

addons/grafana/deployment.yaml

@@ -29,6 +29,8 @@ spec:
               value: "8080"
             - name: GF_AUTH_BASIC_ENABLED
               value: "false"
+            - name: GF_AUTH_DISABLE_LOGIN_FORM
+              value: "true"
             - name: GF_AUTH_ANONYMOUS_ENABLED
               value: "true"
             - name: GF_AUTH_ANONYMOUS_ORG_ROLE