Refactoring of how we run as user

[aledbf]

Test image: quay.io/aledbf/nginx-ingress-controller:0.401

fixes #2781

[aledbf]

@antoineco please check the wip (only the test if we can run setcap and run as root it's missing)

[codecov-io]

Codecov Report

Merging #2825 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2825   +/-   ##
=======================================
  Coverage   47.34%   47.34%           
=======================================
  Files          75       75           
  Lines        5413     5413           
=======================================
  Hits         2563     2563           
  Misses       2525     2525           
  Partials      325      325

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[antoineco]

/lgtm

[k8s-ci-robot]

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