common: ansible_user support in ansible scripts

[grom72]

• ansible_user is undefined on a localhost with a local connection and needs to be always passed with --extra-vars
  https://radeksprta.eu/posts/ansible_user-is-undefined-on-localhost/
• several minor cleanups of Ansible scripts' documentation
• rename the testUser to a new_user

This PR is a replacement for #5955.

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This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.07%. Comparing base (9819db6) to head (9316218).
Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6055   +/-   ##
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  Coverage   70.07%   70.07%           
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  Files         133      133           
  Lines       19563    19563           
  Branches     3261     3261           
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  Hits        13708    13708           
  Misses       5855     5855           

[osalyk]

Reviewed 2 of 3 files at r1, 1 of 3 files at r2, 3 of 3 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @janekmi)