volumes: CVMFS readOnly to true on job pod spec
volumes: CVMFS readOnly to true on job pod spec

* Closes reanahub/reana#260.

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