[feat] Add command line options for showing the system and environments configuration

[kraushm]

Creating the option --show-config listing the system configuration in a readable format.

Closes #306
Closes #215

[codecov-io]

Codecov Report

Merging #633 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #633      +/-   ##
==========================================
+ Coverage    91.5%   91.59%   +0.09%     
==========================================
  Files          72       72              
  Lines        9009     9058      +49     
==========================================
+ Hits         8244     8297      +53     
+ Misses        765      761       -4

Impacted Files	Coverage Δ
reframe/core/systems.py	87.7% <100%> (+2.58%)	⬆️
unittests/test_cli.py	96.3% <100%> (+0.27%)	⬆️
reframe/core/environments.py	90.38% <100%> (+0.38%)	⬆️
reframe/frontend/cli.py	81.01% <100%> (+1.16%)	⬆️
reframe/core/runtime.py	88.11% <100%> (+1.06%)	⬆️

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[vkarak]

I finalized the implementation. Output is YAML-like. I added also an option to show the environment configuration. Still to do is to update the documentation.

[vkarak]

Documentation updated.

[vkarak]

@teojgo @victorusu Can you please review this so that we can merge it?