Refactor kitchen yaml

[julien-lebot]

Refactor the kitchen.yaml so that the platforms are defined in common anchors at the beginning of the file. Many tests could not run because omnitruck.chef.io/ was returning a 404 - so I implemented some logic to check that the platform is supported before creating a platform entry. Another option was to use a static list, but that would require a lot of manual checks to make sure the platforms are still supported. Also, fix multiple anchor definitions of DATADOG.