[feat] Add utility function for retrieving Cray Linux Environment (CLE) information

[vkarak]

I am not doing any special post processing of the values retrieved, since they are well formed strings that you can nicely compare:

if cray_cle_info().release >= '7.0.UP00' and cray_cle_info().patchset < '09':
    # do sth

[codecov-commenter]

Codecov Report

Merging #1419 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1419      +/-   ##
==========================================
+ Coverage   91.81%   91.83%   +0.02%     
==========================================
  Files          82       82              
  Lines       12750    12786      +36     
==========================================
+ Hits        11706    11742      +36     
  Misses       1044     1044              

Impacted Files	Coverage Δ
reframe/utility/os_ext.py	91.06% <100.00%> (+0.52%)	⬆️
unittests/test_utility.py	99.73% <100.00%> (+<0.01%)	⬆️

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[victorusu]

lgtm