Implement execution modification with PATCH

[glatard]

Execution modification in /executions/<executionId> should be done with PATCH instead of PUT.

PATCH is supported in OpenAPI 3.0: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md#path-item-object

Discussed during meeting on 2018-02-14.

[axlbonnet]

I did this using the "JSON Patch" format (RFC 6902) instead of "JSON Merge Patch" (rfc 7396) because :

• JSON Patch requires to introduce a generic json schema to describe all the changes possible on any object. This is easy in OpenAPI and it will be reusable for any PATCH method we would like to use.
• JSON Merge Patch uses the same json schema of the object that needs change, but then the constraints are not the same, especially the required properties that could be absent, and I did not find a way to do that whith OpenAPI.