Fix cgroup creation

[Martchus]

• Pass name to groupv2 function to prevent Failed creating CGroup subtree 'Invalid path at /usr/lib/perl5/vendor_perl/5.26.1/Mojo/IOLoop/ReadWriteProcess/CGroup.pm line 35.
• Improve coding style
• Improve error messages
• Separate temp dir creation from code dealing with cgroups
• See https://progress.opensuse.org/issues/56993

[codecov]

Codecov Report

Merging #2389 into master will decrease coverage by 0.01%.
The diff coverage is 88.23%.

@@            Coverage Diff            @@
##           master   #2389      +/-   ##
=========================================
- Coverage   86.91%   86.9%   -0.02%     
=========================================
  Files         168     168              
  Lines       11160   11187      +27     
=========================================
+ Hits         9700    9722      +22     
- Misses       1460    1465       +5

Impacted Files	Coverage Δ
lib/OpenQA/Worker/Engines/isotovideo.pm	93.83% <88.23%> (-0.75%)	⬇️
lib/OpenQA/WebSockets.pm	92.75% <0%> (-1.45%)	⬇️
lib/OpenQA/Schema/Result/Jobs.pm	86.13% <0%> (-0.82%)	⬇️
lib/OpenQA/WebAPI/Controller/API/V1/JobTemplate.pm	94.18% <0%> (+0.06%)	⬆️
lib/OpenQA/WebAPI/Controller/API/V1/Table.pm	97.56% <0%> (+4.83%)	⬆️

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