[feat] Relax the value syntax of env_vars

[vkarak]

Now the value of environment variables can accept any Python type. ReFrame will invoke str(value) automatically to emit it in a script.

This PR depends on #2648.

Fixes #56.

[codecov-commenter]

Codecov Report

Base: 86.27% // Head: 86.30% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (6232aab) compared to base (425a14d).
Patch coverage: 90.71% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2649      +/-   ##
==========================================
+ Coverage   86.27%   86.30%   +0.03%     
==========================================
  Files          60       60              
  Lines       10992    11088      +96     
==========================================
+ Hits         9483     9570      +87     
- Misses       1509     1518       +9     

Impacted Files	Coverage Δ
reframe/core/systems.py	89.95% <ø> (ø)
reframe/frontend/cli.py	72.30% <0.00%> (-0.12%)	⬇️
reframe/frontend/runreport.py	94.61% <33.33%> (-0.70%)	⬇️
reframe/frontend/statistics.py	96.06% <83.33%> (+0.06%)	⬆️
reframe/core/environments.py	95.12% <90.90%> (-1.28%)	⬇️
reframe/core/variables.py	95.75% <94.28%> (-0.56%)	⬇️
reframe/core/config.py	91.82% <96.15%> (+0.28%)	⬆️
reframe/core/logging.py	77.12% <100.00%> (+0.11%)	⬆️
reframe/core/meta.py	99.08% <100.00%> (ø)
reframe/core/pipeline.py	93.41% <100.00%> (+<0.01%)	⬆️
... and 2 more

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[ekouts]

I am not sure if this is because of the PR. The unittests are failing on daint with:

=================================== FAILURES ===================================
_________________________ test_submit_job_array[slurm] _________________________

make_job = <function make_job.<locals>._make_job at 0x151d2b5dc400>
slurm_only = None
exec_ctx = <reframe.core.systems.SystemPartition object at 0x151d2afc60f0>

    def test_submit_job_array(make_job, slurm_only, exec_ctx):
        job = make_job(sched_access=exec_ctx.access)
        job.options = ['--array=0-1']
        prepare_job(job, command='echo "Task id: ${SLURM_ARRAY_TASK_ID}"')
        submit_job(job)
        job.wait()
        if job.scheduler.registered_name == 'slurm':
            assert job.exitcode == 0
        with open(job.stdout) as fp:
            output = fp.read()
>           assert all([re.search('Task id: 0', output),
                        re.search('Task id: 1', output)])
E           AssertionError: assert False
E            +  where False = all([<_sre.SRE_Match object; span=(15, 25), match='Task id: 0'>, None])

unittests/test_schedulers.py:449: AssertionError
=========================== short test summary info ============================

I will try re-running

[ekouts]

@jenkins-cscs retry daint