PR #13293 added a safety check to not remove a pod directory if the child
volumes directory is not empty. This logic is faulty because kubelet may have
directory structures podUID/volumes/volumeKind/volumeName. E.g.,
`056db95d-50ee-11e5-a2e4-42010af0ba1d/volumes/kubernetes.io~empty-dir/default-token-al3r2`

This change fixes that by properly listing all volumes under a pod.