Get root container stats from cAdvisor

I changed the GetContainerStats function, which will return the root container's stats if the container name is empty. This is useful when the master wants to know machine wide information. e.g. total memory usage of a certain node.
For now, you can look at the loop I put in GetPodInfo: https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/kubelet/kubelet.go#L799

This is a temporary hack, I hope.