html5as-tutorial: Add a custom promise demo

Add a custom promise demo to the hml5as tutorial to show how to write plugin promises.

The custom promise merely checks if a file exists, and is used in the tutorial to check that the index.html file that will be served by the instance exists.

We use the new options added in the slapos.cookbook:promise.plugin recipe by https://lab.nexedi.com/nexedi/slapos/merge_requests/1058 to make creating a custom promise as simple as writing a file and downloading it.

In more details:

• Add a custom promise directly in the SR in promise/check_index_exists.py
• Download this promise.py file
• Add the promise in the instance using the new file option of slapos.cookbook:promise.plugin recipe