[bugfix] Expand the Slurm nodelist using scontrol and not the builtin nodelist_expand() function

[ekouts]

To be honest I like a bit more the original solution of @rsarm , using the utility of scontrol instead of maintaining our own nodelist_expand. I did the minimum changes and haven't updated the unittests to discuss first what you think.

Fixes #2677

[codecov-commenter]

Codecov Report

Base: 86.37% // Head: 86.36% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (6af68b2) compared to base (53aad83).
Patch coverage: 33.33% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2678      +/-   ##
==========================================
- Coverage   86.37%   86.36%   -0.02%     
==========================================
  Files          60       60              
  Lines       11114    11115       +1     
==========================================
- Hits         9600     9599       -1     
- Misses       1514     1516       +2     

Impacted Files	Coverage Δ
reframe/core/schedulers/slurm.py	53.33% <33.33%> (-0.15%)	⬇️
reframe/frontend/executors/policies.py	92.50% <0.00%> (-0.28%)	⬇️

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