refactor force ssl redirect logic

[ElvinEfendi]

Addressing part of #4356 (comment)

The idea is to move dependency on certificate existence to the Lua module so that we don't regenerate Nginx configuration when secret/certificate is added or deleted for the server.

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 #

Special notes for your reviewer:

[ElvinEfendi]

@aledbf this function is the crucial bit here, it is to replace server.SSLCert != nil. Please pay special attention to make sure we aren't introducing a regression.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@73bc2cf). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #4592   +/-   ##
=========================================
  Coverage          ?   58.64%           
=========================================
  Files             ?       88           
  Lines             ?     6736           
  Branches          ?        0           
=========================================
  Hits              ?     3950           
  Misses            ?     2350           
  Partials          ?      436

Impacted Files	Coverage Δ
internal/ingress/controller/template/template.go	79.63% <100%> (ø)

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[ElvinEfendi]

This PR requires #4594.

[ElvinEfendi]

/test pull-ingress-nginx-test

[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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