k8s: return V1Job when creating job
k8s: return V1Job when creating job

* Reverts change done in d660142af4c4eb60d7615fde62a9284a6c47c495
  since it breaks `k8s_delete_job` when called from `watch_jobs`
  (Closes #101).

tiborsimko/reana-job-controller,reanahub/reana-job-controller,diegodelemos/reana-job-controller,reanahub/reana-job-controller,tiborsimko/reana-job-controller,diegodelemos/reana-job-controller