Consider porting from kubeyaml to py-yamler or similar

[squaremo]

As of #976, we run YAML updates through kubeyaml, which is a vast improvement. But it's not very flexible -- kubeyaml has to understand each format we want to be able to update (e.g., FluxHelmReleases), and each kind of update we want to do.

At the minute, we do some analysis of manifests in Go code, and some in kubeyaml, and these have to line up. For example, both have to agree on how FluxHelmReleases encode the image in use in the chart.

If we used a library that supported arbitrary changes to YAMLs, then we'd be able to do the analysis of what to change only in fluxd (and have to change only fluxd to support new formats or kinds of update).

[kingdonb]

In Flux v2, Kustomize's kyaml setters feature is used instead of manipulating YAML directly inside of Flux components.

Closing.