don't mount aufs or overlayfs in overlayfs

overlayfs can be mounted in overlayfs, but has the same whiteout problem as in
aufs. aufs mounts but has strange behavior.
clean up nested warden specs

* no longer install ruby with rvm; the ruby in the rootfs should be fine. this
  shaves many minutes off running the tests.
* with DEBUG=true, stream the output of the commands that are being run in the
  container. this gives more information when it fails than '1 should be 0'.
* destroy the outer container in cleanup
* don't trash dev/* in the rootfs. this only affects overlayfs in overlayfs,
  which will no longer be the case for these tests, as it will properly fall
  back to the copying + bind-mount mechanism.
only print exit status if debugging