Vendoring of Helm charts should be automated with "buildchain" targets

[gdemonet]

Component: build, charts

Why this is needed:

Everytime one wants to update a vendored Helm chart, a manual process is required to:

• find the source of the Helm chart, to further issue a helm fetch $chart_src
• re-apply patches to these charts in-tree
• re-render the right SLS from the chart and its options using the right command-line (which can be found in the last commit message updating said SLS...)

This is cumbersome and error-prone.

What should be done:

Add doit targets for:

• fetching and vendoring a Helm chart (with source + version stored in the buildchain package), applying patches as required (they would need to be managed differently than today, I guess)
• rendering the corresponding SLS formula(s)

[gdemonet]

One approach to consider: instead of patching the vendored chart + applying some magic in our render script, try to "just" run helm template and use kustomize to modify the resulting YAML, and finally convert this intermediate YAML into a proper SLS.
Also, make sure we store all intermediate files in the _build directory for easier inspection.
If all this works, we could even consider not vendoring Helm charts at all.

[gdemonet]

Relates to #1449