[stable/grafana-agent] Support existing ConfigMap.

[DandyDeveloper]

Signed-off-by: Aaron Layfield aaron.layfield@gmail.com

What this PR does / why we need it:

This enabled an option for using an existing ConfigMap

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes #164

Special notes for your reviewer:

@davosian does this fit your requirements?

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• [x] DCO signed
• [x] Chart Version bumped
• [x] Variables are documented in the README.md
• [x] Title of the PR starts with chart name (e.g. [stable/mychartname])

[DandyDeveloper]

Merging.