Make workflow execution creation idempotent

We need to make the process of creating workflow execution idempotent. The proposed change is to allow to pass workflow execution ID to the endpoint /v2/executions when we make a POST request:

POST /v2/executions

{
  "id": "a-b-c-d",
  ...
}

so that if an execution with such ID exists the endpoint will just return its state (a JSON structure), and if it doesn't exist it will create an execution and start the corresponding workflow.

The change is fully backwards compatible: if ID is not passed the endpoint works the same way as before.

More details are described in the etherpad:
https://etherpad.openstack.org/p/mistral-execution-create