k8s: allow automounting of CVMFS repositories

[mdonadoni]

Closes reanahub/reana-commons#423

[mdonadoni]

CI should pass after releasing new alpha version of r-commons and bumping version in setup.py

[tiborsimko]

PVC well attempted locally:

$ kubectl get pvc
NAME          STATUS    VOLUME   CAPACITY   ACCESS MODES   STORAGECLASS   AGE
reana-cvmfs   Pending                                      reana-cvmfs    111s

[codecov]

Codecov Report

Merging #540 (0fe6c86) into master (15e2289) will increase coverage by 0.74%.
The diff coverage is 55.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #540      +/-   ##
==========================================
+ Coverage   71.48%   72.23%   +0.74%     
==========================================
  Files          15       15              
  Lines        1403     1390      -13     
==========================================
+ Hits         1003     1004       +1     
+ Misses        400      386      -14     

Files	Coverage Δ
reana_workflow_controller/k8s.py	88.79% <0.00%> (+7.54%)	⬆️
reana_workflow_controller/workflow_run_manager.py	81.69% <62.50%> (+1.31%)	⬆️