sort auth proxy headers from configmap

[ElvinEfendi]

What this PR does / why we need it:

Fixing flaky order dependent test introduced in #4550:

--- FAIL: TestBuildAuthProxySetHeaders (0.00s)
    template_test.go:466: Expected 
        '[proxy_set_header 'header1' 'value1'; proxy_set_header 'header2' 'value2';]'
        but returned 
        '[proxy_set_header 'header2' 'value2'; proxy_set_header 'header1' 'value1';]'
W0924 19:02:38.196548   22128 template.go:120] unexpected error cleaning template: fork/exec /ingress-controller/clean-nginx-conf.sh: no such file or directory

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

Special notes for your reviewer:

[aledbf]

/lgtm

[aledbf]

@ElvinEfendi is a map, not a list, you need to create an array with the keys, sort that and iterate the sorted list

[codecov-io]

Codecov Report

Merging #4596 into master will increase coverage by 1.29%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4596      +/-   ##
==========================================
+ Coverage   58.58%   59.88%   +1.29%     
==========================================
  Files          88       88              
  Lines        6735     7008     +273     
==========================================
+ Hits         3946     4197     +251     
- Misses       2352     2360       +8     
- Partials      437      451      +14

Impacted Files	Coverage Δ
internal/ingress/controller/template/template.go	82.78% <100%> (+3.62%)	⬆️
internal/watch/file_watcher.go	80.76% <0%> (-3.85%)	⬇️
internal/ingress/metric/collectors/process.go	90.42% <0%> (+2.12%)	⬆️

---

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[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, ElvinEfendi

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• OWNERS [ElvinEfendi,aledbf]

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