[bugfix] Fix duplicate module commands emitted while executing test cases

[vkarak]

• This fixes the problem with the duplicate module commands. Essentially, we revert back to the original solution of deep-copying the environments and the partitions. Environments are not immutable, since they hold a state. Similarly, since a partition holds a reference to its local environment, it's not immutable as well.
• This PR removes also the stale Sandbox class.

Fixes #737.

[codecov-io]

Codecov Report

Merging #740 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #740      +/-   ##
==========================================
+ Coverage   91.73%   91.74%   +<.01%     
==========================================
  Files          77       76       -1     
  Lines        9527     9509      -18     
==========================================
- Hits         8740     8724      -16     
+ Misses        787      785       -2

Impacted Files	Coverage Δ
unittests/test_fields.py	97.22% <ø> (-0.2%)	⬇️
reframe/frontend/executors/__init__.py	97.19% <100%> (-0.03%)	⬇️
reframe/core/config.py	84.48% <0%> (+1.72%)	⬆️

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[vkarak]

@jenkins-cscs retry daint kesch