Add staticfiles dirs to devstack settings.

To handle static files for working with XML courses on devstack.
@jarv 
`DATA_DIR` is a `path.py` object, which means you can run `if DATA_DIR.isdir():` instead of `if os.path.isdir(DATA_DIR)`. It does the same thing, but it's cleaner.

Also, do you intend to check if `DATA_DIR` exists? (You can run `DATA_DIR.exists()`)