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/unity.py

@@ -17,7 +17,7 @@ Note that it was assumed that:
 
 The script will need to be updated if someday those conditions are not met."""
 
-with releasedata.ProductData("unity", cumulative_update=True) as product_data:
+with releasedata.ProductData("unity") as product_data:
     response = http.fetch_url("https://unity.com/releases/editor/qa/lts-releases")
     soup = BeautifulSoup(response.text, features="html5lib")