Add support for external databases not deployed by subcharts

[jochbru]

This helm chart should have support for external databases that have not been deployed by thge Bitnami subcharts. This enables support for use with operated mysql workloads.

[CodeBleu]

@jochbru @jacobw I would like to be able to use LibreNMS in K8S, but need a way to have it use an external MySQL DB. How would you recommend doing this?

[jacobw]

Sorry for the slow reply. The helm chart needs to be updated to support that. I'd like to do that but don't have a ETA sorry. PRs welcome.

[mkuurstra]

I made a PR for this: #94