fix(sessions): get the session status from the pod state

[Alputer]

Closes reanahub/reana-ui#408

[codecov]

Codecov Report

Attention: Patch coverage is 15.00000% with 17 lines in your changes missing coverage. Please review.

Project coverage is 75.95%. Comparing base (7896c61) to head (49cf295).

Files with missing lines	Patch %	Lines
reana_workflow_controller/k8s.py	9.09%	10 Missing ⚠️
reana_workflow_controller/rest/workflows.py	22.22%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #611      +/-   ##
==========================================
- Coverage   76.61%   75.95%   -0.66%     
==========================================
  Files          17       17              
  Lines        1847     1867      +20     
==========================================
+ Hits         1415     1418       +3     
- Misses        432      449      +17     

Files with missing lines	Coverage Δ
reana_workflow_controller/rest/workflows.py	70.48% <22.22%> (-1.56%)	⬇️
reana_workflow_controller/k8s.py	78.23% <9.09%> (-5.60%)	⬇️

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