refactor(tests): use constants for referencing images and ports

[programmer04]

What this PR does / why we need it:

Initial refactor - do not use magic strings and numbers to refer images and their ports used in tests.

Which issue this PR fixes:

This is a part of implementing e2e tests that verifies proper handling of EndpointSlices #4026

Special notes for your reviewer:

This is the part of implementing strategy proposed in this one too big PR #4084.

In test/integration/tlsroute_test.go:765 const tlsEchoPort and its usage is a little bit mysterious to me. I backchannelled it to the author to gain more understanding (and maybe handle in different PR).

Labelled with ci/run-e2e to be sure, result has to be examined.

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[team-k8s-bot]

E2E (targeted) tests were started at https://github.com/Kong/kubernetes-ingress-controller/actions/runs/5090073830

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.1 ⚠️

Comparison is base (753e91f) 60.0% compared to head (931b591) 60.0%.

❗ Current head 931b591 differs from pull request most recent head 8b1f91f. Consider uploading reports for the commit 8b1f91f to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4088     +/-   ##
=======================================
- Coverage   60.0%   60.0%   -0.1%     
=======================================
  Files        151     151             
  Lines      16612   16612             
=======================================
- Hits        9982    9981      -1     
- Misses      5999    6000      +1     
  Partials     631     631             

see 3 files with indirect coverage changes

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[programmer04]

For ci/run-e2e on GKE image that does not include #4071 (from yesterday was used, thus GKE results are ignored. For Kind everything is 🟢