Allow custom CA certificate when flag --api-server is specified

[aledbf]

What this PR does / why we need it:

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #4783

Special notes for your reviewer:

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf

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[codecov-io]

Codecov Report

Merging #4807 into master will decrease coverage by 0.07%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##           master    #4807      +/-   ##
==========================================
- Coverage   58.59%   58.51%   -0.08%     
==========================================
  Files          88       88              
  Lines        6702     6714      +12     
==========================================
+ Hits         3927     3929       +2     
- Misses       2349     2359      +10     
  Partials      426      426

Impacted Files	Coverage Δ
internal/ingress/controller/controller.go	50.49% <ø> (ø)	⬆️
cmd/nginx/main.go	6.21% <11.11%> (-0.29%)	⬇️
cmd/nginx/flags.go	85.94% <60%> (-0.73%)	⬇️
internal/watch/file_watcher.go	80.76% <0%> (-3.85%)	⬇️

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[aledbf]

/retest