Move creation of open api clients to reana-commons

[dinosk]

We can centralize the creation of open api clients (from server, client by moving them to reana-commons)

Same thing could be done for the open api tests (like in job-controller, workflow-controller).
Note there are two tests for the openapi, one for validation of the schema, and one to verify that the schema is not outdated.

[diegodelemos]

Addressed by #47. Central validation of OpenAPI specs and central testing if the spec is outdated still to be done.

[diegodelemos]

Addressed by #71, reanahub/reana-server#115, reanahub/reana-client#214, reanahub/reana-workflow-engine-cwl#76, reanahub/reana-workflow-engine-serial#55 and #71 during Live-Mode sprint.

---

What remains:

• Use central OpenAPI client factory inside Job-Controller (relevant RFC openapi: generation of OpenAPI spec differs from rest of REANA components reana-job-controller#106)
• Extract Click command to generate OpenAPI specs, should be addressed in CLI: generate OpenAPI specs for generic components #76
• Centralize tests for the OpenAPI specs, validation of the schema and testing if newly generated spec is outdated.