This patch removes pkg/util/mount completely, and replaces it with the
mount package now located at k8s.io/utils/mount. The code found at
k8s.io/utils/mount was moved there from pkg/util/mount, so the code is
identical, just no longer in-tree to k/k.
With bug #27653, kubelet could remove mounted volumes and delete user data.
The bug itself is fixed, however our trust in kubelet is significantly lower.
Let's add an extra version of RemoveAll that does not cross mount boundary
(rm -rf --one-file-system).
It calls lstat(path) three times for each removed directory - once in
RemoveAllOneFilesystem and twice in IsLikelyNotMountPoint, however this way
it's platform independent and the directory that is being removed by kubelet
should be almost empty.
