Properly integrate chart rendering in buildchain

[NicolasT]

https://github.com/scality/metalk8s/pull/1445 introduces a Helm chart that's rendered using a simple script and then checked into the repo. Instead, this should be done by the buildchain (creating the chart.sls file), such that this functionality can easily be reused for other charts.

[NicolasT]

So, I'm no longer sure we should do this.

The rationale is as follows: when running the rendering at build time, it becomes very difficult to 'know' what exactly changed when a PR updating a chart version, changing values,... is submitted.

Enforcing the generated files to be checked in makes the actual change much more obvious to reviewers.

[slaperche-scality]

Maybe we can do something like what have been done with Go boilerplate: have a lint step ensuring that the codegen was run and the resulting files commited.