Factor out WebHDFS URL to app config

fixes #20 
it feels weird to have a URL fragments in long-lived global vars like this. How about

    WEBHDFS_URL_PATTERN = '{}/webhdfs/v1/\\{\\}?user.name={}&op=OPEN'.format(
        app.config['WEBHDFS_URL'], app.config['WEBHDFS_USER'])

so that the global is at least self-contained?