Fix file handler leak in computemgtd

[mw0000001]

Fix for closing filehandle of tempfile and delete it. (mkstemp, is not closing os filehandle automatically)
computemgtd was running into a too many open files issue, every ~8 days. Which made compute node be restarted.

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[codecov]

Codecov Report

Merging #421 (cb75870) into develop (8eca5d4) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #421      +/-   ##
===========================================
+ Coverage    81.58%   81.59%   +0.01%     
===========================================
  Files           12       12              
  Lines         1732     1733       +1     
===========================================
+ Hits          1413     1414       +1     
  Misses         319      319              

Impacted Files	Coverage Δ
src/slurm_plugin/computemgtd.py	61.71% <100.00%> (+0.30%)	⬆️

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[demartinofra]

Good catch and thanks for the patch!!!

[demartinofra]

It's better to store the reference to NamedTemporaryFile() in the class attributes otherwise when tf gets out of scope the file is deleted. I will create a separate PR to improve this.

[EDIT] apparently the temp file is only used in _get_config, I'll just move its creation in there then.