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.
Just read the TODO in ``getContainerIdFromName()``. It seems that we should not use this function any more. Is there any way to provide a function like ``getContainerIdFromPidIdAndName()``? Right now, the only way I can imagine to get the docker Id from pod Id + container name is to use ``GetPodInfo()`` and find the docker id in the returned api.PodInfo.