perf(k8s): avoid fetching secrets multiple times (#456)

[mdonadoni]

Closes #455

[tiborsimko]

It'll be good to release pytest-reana first, then bump its version here in this PR, so that the CI checks would pass.

A suggestion for the release news: instead of:

perf(k8s): avoid fetching secrets multiple times

we could perhaps say "avoid repetitive fetching of user secrets" or "avoid extraneous fetching..." or "avoid unnecessary refetching..." etc for the message clarity?

[codecov]

Codecov Report

Attention: Patch coverage is 85.57692% with 15 lines in your changes missing coverage. Please review.

Project coverage is 37.10%. Comparing base (273b72d) to head (4816f90).
Report is 2 commits behind head on master.

Files	Patch %	Lines
reana_commons/k8s/secrets.py	85.41%	14 Missing ⚠️
reana_commons/version.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #456      +/-   ##
==========================================
+ Coverage   34.84%   37.10%   +2.25%     
==========================================
  Files          26       26              
  Lines        1650     1652       +2     
==========================================
+ Hits          575      613      +38     
+ Misses       1075     1039      -36     

Files	Coverage Δ
reana_commons/k8s/kerberos.py	92.00% <100.00%> (+92.00%)	⬆️
reana_commons/version.py	0.00% <0.00%> (ø)
reana_commons/k8s/secrets.py	82.75% <85.41%> (+12.93%)	⬆️