Apply OpenAPI principles to ELSA design

[christophe-dupriez]

Starting from data and operations of ELSA, align ELSA API on OpenAPI principles ( https://www.openapis.org/ ) and produce the JSON description of current ELSA API. Check its coherence with desired actions and the data model (by the way, finish drawing the data model using GraphViz)

Evaluate Python OpenAPI Web Frameworks. Those frameworks may ask for Python3. Switching to Python 3: https://www.pluralsight.com/courses/python-2-to-python-3