Support serve_js and serve_css settings

Problem/Motivation

drush cr dies to a azure://css is not a directory. error when serve_js or serve_css flysystem setting is enabled.

Steps to reproduce

Enable serve_js and serve_css settings:

$schemes = [
  'azure' => [
    'driver' => 'azure',
    'config' => [
      'name' => 'xxx',
      'key' => 'xxx',
      'container' => 'xxx',
      'endpointSuffix' => 'core.windows.net',
      'protocol' => 'https',
    ],
    'cache' => TRUE,
    'serve_js' => TRUE,
    'serve_css' => TRUE,
  ],
];

$settings['flysystem'] = $schemes;