Add subdirectory-output support and change official releases

This PR does a few things:

* Add slice option `dockerfile_subdirectory` for
  https://github.com/tensorflow/tensorflow/pull/24180, a slice option that
  places any Dockerfile using the slice into a subdirectory of the
  Dockerfiles directory.
* Rearrange releases and slices slightly. The `devel` images are now
  going to be built nightly, to best support master, and are tagged
  `devel-*`.