[feat] Add high-level API for container platforms

[rsarm]

Fixes https://jira.cscs.ch/browse/UES-186

[vkarak]

@rsarm Rename also the containerplatform.py to containers.py.

[codecov-io]

Codecov Report

Merging #787 into master will increase coverage by 0.22%.
The diff coverage is 88%.

@@            Coverage Diff             @@
##           master     #787      +/-   ##
==========================================
+ Coverage   91.51%   91.74%   +0.22%     
==========================================
  Files          76       80       +4     
  Lines        9550    10454     +904     
==========================================
+ Hits         8740     9591     +851     
- Misses        810      863      +53

Impacted Files	Coverage Δ
reframe/core/exceptions.py	81.63% <100%> (-0.4%)	⬇️
reframe/core/containerplatform.py	81.81% <81.81%> (ø)
unittests/test_containerplatform.py	96.66% <96.66%> (ø)
unittests/test_policies.py	98.86% <0%> (-0.32%)	⬇️
unittests/resources/settings.py	100% <0%> (ø)	⬆️
reframe/frontend/dependency.py	93.47% <0%> (ø)
...s/resources/checks_unlisted/dependencies/normal.py	100% <0%> (ø)
... and 4 more

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[vkarak]

@rsarm Is this still a WIP?

[vkarak]

My only comment is should we call it container_platform or container_runtime? @teojgo @rsarm What do you think?