Retrieve machine spec from cAdvisor

This is the third piece of #448. It allows kubelet to retrieve machine spec (number of cores, total memory capacity.) from cAdvisor. The naming here is a bit confusing now: GetMachineInfo() and GetContainerInfo() returns info.ContainerInfo; while GetMachineSpec() returns info.MachineInfo. Probably we should rename MachineInfo to MachineSpec in cAdvisor? WDYT @vmarmol?