Use config file instead of args for kas

What does this MR do?

This is the first iteration of #2372 (closed), just moving kas to use a configmap instead of command line arguments.

It currently only adds exactly the same configuration as was there before. MVC customizability is achieved via editing the config map directly.

We also know that we will need to expose the following options very soon:

1. websockets
2. sentry
3. google cloud profiler
4. redis

Related issues

• #2372 (closed)
• gitlab-org/cluster-integration/gitlab-agent#16 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com: Pipeline with KAS_ENABLED = 1 https://gitlab.com/gitlab-org/charts/gitlab/-/pipelines/216946692

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• [-] Integration tests added to GitLab QA: I don't think we have any yet)
• [-] Equivalent MR/issue for omnibus-gitlab opened: kas already uses a config file over there

Test plan

1. Used the existing cluster kas-chart-qa in gitlab-paas GCP project, with an existing GitLab release with kas enabled
2. Upgraded to the version of the chart in this MR
3. Tested:
   • the kas pods got upgraded and started correctly
   • the in-cluster deployment resource looked correct
   • the configmap got applied and looked correct
   • gitops was still working in an existing manifest project