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.