Set NGINX Plus default image

[sjberman]

Problem: The default NGINX Plus image wasn't being set properly.

Solution: Set the default if running Plus.

Closes #3920

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

Set correct default NGINX Plus image when not specified by user.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.80%. Comparing base (4e3dc46) to head (b2995e0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3919      +/-   ##
==========================================
+ Coverage   86.78%   86.80%   +0.02%     
==========================================
  Files         128      128              
  Lines       16500    16503       +3     
  Branches       62       62              
==========================================
+ Hits        14319    14325       +6     
+ Misses       2001     1999       -2     
+ Partials      180      179       -1     

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