nixos/kubernetes: dashboard lockdown

Kubernetes dashboard currently has cluster admin permissions,
which is not recommended.

- Renamed option "services.kubernetes.addons.dashboard.enableRBAC" to "services.kubernetes.addons.dashboard.rbac.enable"
- Added option "services.kubernetes.addons.dashboard.rbac.clusterAdmin", default = false.
- Setting recommended minimal permissions for the dashboard in accordance with https://github.com/kubernetes/dashboard/wiki/Installation
- Updated release note for 18.09.

nixos/modules/rename.nix

@@ -34,6 +34,7 @@ with lib;
     (mkRenamedOptionModule [ "services" "kubernetes" "apiserver" "admissionControl" ] [ "services" "kubernetes" "apiserver" "enableAdmissionPlugins" ])
     (mkRenamedOptionModule [ "services" "kubernetes" "apiserver" "address" ] ["services" "kubernetes" "apiserver" "bindAddress"])
     (mkRemovedOptionModule [ "services" "kubernetes" "apiserver" "publicAddress" ] "")
+    (mkRenamedOptionModule [ "services" "kubernetes" "addons" "dashboard" "enableRBAC" ] [ "services" "kubernetes" "addons" "dashboard" "rbac" "enable" ])
     (mkRenamedOptionModule [ "services" "logstash" "address" ] [ "services" "logstash" "listenAddress" ])
     (mkRenamedOptionModule [ "services" "mpd" "network" "host" ] [ "services" "mpd" "network" "listenAddress" ])
     (mkRenamedOptionModule [ "services" "neo4j" "host" ] [ "services" "neo4j" "listenAddress" ])