[bugfix] Fix reading of 'logdir' setting from system config

[vkarak]

Also:

• The logdir system config attribute is deprecated and replaced by the
  perflogdir.

• Documentation needs to be updated.

Fixes #419.

[codecov-io]

Codecov Report

Merging #445 into master will increase coverage by <.01%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #445      +/-   ##
==========================================
+ Coverage   91.23%   91.23%   +<.01%     
==========================================
  Files          70       70              
  Lines        8611     8625      +14     
==========================================
+ Hits         7856     7869      +13     
- Misses        755      756       +1

Impacted Files	Coverage Δ
unittests/resources/settings.py	100% <ø> (ø)	⬆️
reframe/core/systems.py	85% <100%> (ø)	⬆️
unittests/test_config.py	97.53% <100%> (+0.09%)	⬆️
reframe/frontend/cli.py	78.43% <100%> (+0.39%)	⬆️
unittests/test_runtime.py	100% <100%> (ø)	⬆️
reframe/core/config.py	82.45% <66.66%> (-0.28%)	⬇️
reframe/core/runtime.py	87.09% <85.71%> (-0.2%)	⬇️

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