Preserve field order when updating kustomization.yaml file

[jtdoepke]

This commit causes argocd-image-updater to preserve the order of fields in the kustomization.yaml file when updating the image(s). Doing so will minimize git diffs in the case that other tools also edit the kustomization.yaml file.

Fixes #502

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 30.76923% with 9 lines in your changes missing coverage. Please review.

Project coverage is 67.84%. Comparing base (2bf4b0a) to head (a98c209).

Files	Patch %	Lines
pkg/argocd/git.go	30.76%	4 Missing and 5 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #666      +/-   ##
==========================================
- Coverage   67.98%   67.84%   -0.15%     
==========================================
  Files          31       31              
  Lines        3124     3135      +11     
==========================================
+ Hits         2124     2127       +3     
- Misses        856      860       +4     
- Partials      144      148       +4     

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[jtdoepke]

The build_image job failed, but I'm pretty sure that's not due to my changes.

[chengfang]

@jtdoepke can you also fix the DCO failure with sign-off?

[chengfang]

@jtdoepke thanks!