make cadvisor port configurable on the kubelet

The [cadvisor quickstart guide](https://github.com/google/cadvisor#quick-start-running-cadvisor-in-a-docker-container) recommends port 8080 but the kubelet looks for cadvisor on the hardcoded port 4194. This decision is also not documented anywhere and could lead to confusion. See GoogleCloudPlatform/heapster#15. It would be nice to have the option to configure the port kubelet looks for cadvisor on.