Improve scripts execution orchestration (#299)

Until now products could declare multiple auto-update methods, but they all had to be of the same kind.
For example if you used the git auto-update method, you could not use an additional github_releases or custom auto-update method.
This is an issue as it prevents us to extend the auto-update process, for example by having a product using the 'git' auto-update method to retrieve all the versions, and a custom script to retrieve support and EOL dates.

This improve the scripts execution orchestration to be able to support auto configurations using a mix of methods, meaning:

- multiple kind of methods, such as git and github_release,
- or multiple custom methods.

A side-effect of those changes is that now a failure in a generic script does not cancel the update of subsequent products.

Another side-effect, unwanted this time, is that now custom scripts managing multiple products, such as apple.py, are now executed multiple times instead of once.

src/aws-lambda.py

@@ -28,8 +28,6 @@ with releasedata.ProductData("aws-lambda") as product_data:
             identifier = cells[identifier_index].get_text().strip()
 
             date = product_frontmatter.get_release_date(identifier)  # use the product releaseDate if available
-            if date is None:
-                date = product_data.get_previous_version(identifier).date()  # else use the previously found date
             if date is None:
                 date = dates.today()  # else use today's date