Set a default temporary and public file paths

[herbdool]

From https://pantheon.io/docs/tmp#default-temporary-path, we should add something like this to settings.php:

/**
 * Define appropriate location for tmp directory
 */
if (isset($_ENV['PANTHEON_ENVIRONMENT'])) {
  $conf['file_temporary_path'] = $_SERVER['HOME'] .'/tmp';
}

And should do the same for public files and show how to to do it for private file directory.

[jenlampton]

I think this should be possible since 1.16.