[feat] Support command substitution in variable expansion

[vkarak]

• Extend expandvars() to support command substitutions.
• Replace almost all usages of os.path.expandvars() with our version.

Fixes #682.

I still need to update a bit the documentation. I believe this is a feature rather than a bugfix.

[codecov-io]

Codecov Report

Merging #683 into master will increase coverage by 0.05%.
The diff coverage is 94.33%.

@@            Coverage Diff             @@
##           master     #683      +/-   ##
==========================================
+ Coverage   91.82%   91.87%   +0.05%     
==========================================
  Files          77       77              
  Lines        9403     9444      +41     
==========================================
+ Hits         8634     8677      +43     
+ Misses        769      767       -2

Impacted Files	Coverage Δ
reframe/core/environments.py	88.67% <100%> (ø)	⬆️
reframe/utility/os_ext.py	90.25% <100%> (+0.6%)	⬆️
reframe/frontend/cli.py	81.01% <100%> (ø)	⬆️
unittests/test_utility.py	99.03% <100%> (+0.06%)	⬆️
reframe/core/config.py	84.48% <57.14%> (+1.87%)	⬆️

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