refactor(secrets): adapt to reana-commons secret-handling changes (#583)

[mdonadoni]

Closes reanahub/reana-commons#455

[mdonadoni]

TODO:

• Test GitLab authentication flow on DEV (and possibly write a test for it)
• Make necessary changes to rebase on top of feat(sessions): expose user secrets in interactive sessions (#591) #591

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 6 lines in your changes missing coverage. Please review.

Project coverage is 75.13%. Comparing base (fb9923c) to head (d9512fb).
Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
reana_workflow_controller/rest/utils.py	16.66%	5 Missing ⚠️
reana_workflow_controller/consumer.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #583      +/-   ##
==========================================
+ Coverage   74.28%   75.13%   +0.85%     
==========================================
  Files          15       15              
  Lines        1466     1476      +10     
==========================================
+ Hits         1089     1109      +20     
+ Misses        377      367      -10     

Files with missing lines	Coverage Δ
reana_workflow_controller/k8s.py	88.97% <100.00%> (ø)
reana_workflow_controller/workflow_run_manager.py	81.27% <100.00%> (+0.08%)	⬆️
reana_workflow_controller/consumer.py	55.84% <90.00%> (+10.57%)	⬆️
reana_workflow_controller/rest/utils.py	81.06% <16.66%> (-0.73%)	⬇️

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