[feat] Better logging of framework activities, especially test loading

[vkarak]

• The initialization of the framework and the loading of checks is much better logged now. The test loading and filtering is logged at the verbose level.
• A new debug level, DEBUG2, was introduced. All messages from the execution policies and the actual OS commands that are being executed are logged in this level.
• The default level for the logger objects is now set to "undefined." This allows for the seamless addition of new debug levels in the future.
• The stdout and stderr markers in errors for spawned processes have changed.

Fixes #1046.

It also addresses #795, since by increasing verbosity you can now see what files are being loaded and how many tests.

[codecov-io]

Codecov Report

Merging #1571 into master will decrease coverage by 0.04%.
The diff coverage is 85.41%.

@@            Coverage Diff             @@
##           master    #1571      +/-   ##
==========================================
- Coverage   88.04%   87.99%   -0.05%     
==========================================
  Files          44       44              
  Lines        6899     6958      +59     
==========================================
+ Hits         6074     6123      +49     
- Misses        825      835      +10     

Impacted Files	Coverage Δ
reframe/core/schedulers/pbs.py	64.70% <ø> (+0.28%)	⬆️
reframe/core/schedulers/torque.py	19.40% <0.00%> (-1.19%)	⬇️
reframe/core/settings.py	100.00% <ø> (ø)
reframe/core/schedulers/slurm.py	53.61% <38.46%> (-0.43%)	⬇️
reframe/core/logging.py	83.38% <80.00%> (-0.06%)	⬇️
reframe/frontend/cli.py	79.53% <83.87%> (+0.71%)	⬆️
reframe/core/config.py	90.57% <87.50%> (-0.29%)	⬇️
reframe/frontend/loader.py	91.17% <88.88%> (-0.23%)	⬇️
reframe/core/pipeline.py	92.00% <90.00%> (-0.45%)	⬇️
reframe/core/exceptions.py	93.43% <100.00%> (+0.19%)	⬆️
... and 10 more

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[ekouts]

Lgtm

[vkarak]

@jenkins-cscs retry daint