[bugfix] Do not require a configuration to be resolved always at system level

[vkarak]

Fixes #1453.

[vkarak]

@jenkins-cscs retry daint

[teojgo]

Other than the comments that you have posted I have tested and it looks good.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@84ce07d). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #1508   +/-   ##
=========================================
  Coverage          ?   91.51%           
=========================================
  Files             ?       83           
  Lines             ?    13198           
  Branches          ?        0           
=========================================
  Hits              ?    12078           
  Misses            ?     1120           
  Partials          ?        0           

Impacted Files	Coverage Δ
reframe/core/config.py	90.82% <100.00%> (ø)
reframe/core/systems.py	88.31% <100.00%> (ø)
reframe/frontend/cli.py	79.60% <100.00%> (ø)
unittests/test_config.py	100.00% <100.00%> (ø)

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