Improve Application updates when write-back method is ArgoCD

[aaguilartablada]

When the write-back method is ArgoCD, the Image Updater sends an update to the ArgoCD Application on each iteration. No matter whether the new image has already been set or not, it always sends the update. In a situation where autoSync is disabled, this means that it continuously sends updates until somebody syncs the Application.

To reduce updates and improve resource usage, this pull request slightly changes the behavior. The Image Updater will check if the new image is already set in the Application source to apply the update or ignore it.

We have tested the changes in our ArgoCD instance to check the feature and you can see the results in the image below. Updates have been reduced. Once they are applied, updates disappear. CPU and memory usage have been improved.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 27.38095% with 61 lines in your changes missing coverage. Please review.

Project coverage is 73.61%. Comparing base (8076d20) to head (0f06fcb).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/argocd/argocd.go	15.25%	46 Missing and 4 partials ⚠️
pkg/argocd/update.go	56.00%	8 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #965      +/-   ##
==========================================
- Coverage   74.64%   73.61%   -1.04%     
==========================================
  Files          31       31              
  Lines        3913     3991      +78     
==========================================
+ Hits         2921     2938      +17     
- Misses        856      910      +54     
- Partials      136      143       +7     

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