Update build_workflow to support DataImport.Workers

The DataImport.build_workflow method needs to do the following:

1. add an element init task, just like for Elements
2. add one task per worker run
3. each worker run will have a slug and a build_task method

The Worker run slug is the slug of the worker version concatenated with the 6 first chars of the worker run (or worker version ) id

The build_task method must return a dictionnary with the following keys:

• image is null
• command is the worker version command from its config
• artifact is the uuid of the docker image
• parents is the list of parents worker run slugs