feat(promtail-mixin): improve standalone usage

[Zaunei]

What this PR does / why we need it:

This PR improves the usage of the promtail-mixin in setups where no Loki is installed and writes metrics in the same tenant.

With the default Helm installation of promtail, the job label (job=~"promtail-metrics") does not match the currently hard-coded label(job=~"$namespace/promtail"), so the latency graphs will not work. Since the selectors will always differ slightly from setup to setup, I made them configurable.

Added:

• Extend the Makefile to compile promtail-mixins
• Compiled promtail-mixins

Changed:

• Make Dashboard name and label selectors configurable
• Adopt loki-mixin file structure/ Jsonnet stlye

Which issue(s) this PR fixes:

No issue created, as I consider this to be a non-breaking fix and improvement.

Special notes for your reviewer:

In order not to cause any braking changes I have taken over the values used before. Checklist

• [x] Reviewed the CONTRIBUTING.md guide (required)
• [ ] Documentation added
• [ ] Tests updated
• [x] CHANGELOG.md updated
• [ ] Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md