Automatic merge from submit-queue
kubelet/cadvisor: Refactor cadvisor disk stat/usage interfaces.
basically
1) cadvisor struct will know what runtime the kubelet is, passed in via additional argument to New()
2) rename cadvisor wrapper function to DockerImagesFsInfo() to ImagesFsInfo() and have linux implementation choose a label based on the runtime inside the cadvisor struct
2a) mock/fake/unsupported modified to take the same additional argument in New()
3) kubelet's wrapper for the cadvisor wrapper is renamed in parallel
4) make all tests use new interface
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