Code review comment for ~rharding/charm-grafana:add-mysql-datasource

I've updated the interface and updated this MP removing much of the lint cleanup previously done.

I can look into mojo stuff there but that'll be a longer road to hoe as I've not gotten that setup and since they're not currently working...I'd prefer to add a test/support for gypsy-danger as part of something that was working to test current Grafana/Prometheus if required.