checkpoint: clean up orphaned checkpoints.
checkpoint: clean up orphaned checkpoints.

If the checkpointer crashes during `writeAndAtomicRename()` then it
may leave temporary files that start with `.`. This checks for and
removes them.

Also changes `writeAndAtomicRename()` to use `ioutil.TempFile()`.

kubernetes-incubator/bootkube,dghubble/bootkube,kubernetes-incubator/bootkube,kubernetes-incubator/bootkube,diegs/bootkube,diegs/bootkube,dghubble/bootkube,diegs/bootkube,dghubble/bootkube