[ci] Add option to disable CSCS CI tests

[teojgo]

Closes #528

[codecov-io]

Codecov Report

Merging #648 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #648      +/-   ##
==========================================
- Coverage    91.7%   91.68%   -0.03%     
==========================================
  Files          74       74              
  Lines        9256     9256              
==========================================
- Hits         8488     8486       -2     
- Misses        768      770       +2

Impacted Files	Coverage Δ
reframe/core/config.py	82.6% <0%> (-1.74%)	⬇️

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[teojgo]

@jenkins-cscs retry none