Code review comment for ~rharding/interface-grafana-source:add-mysql-support

To support this I've pushed up my built grafana charm with the interface and tested it via:

    juju deploy mysql; juju deploy cs:~rharding/gypsy-danger-3; juju deploy cs:~rharding/grafana-1
    juju relate mysql:db gypsy-danger; juju relate gypsy-danger:grafana-source grafana
    juju expose grafana

And validated I can pull up the list of datasources and successfully "Save and test" the mysql datasource that's added.

Then I made sure it didn't regress promtheus by doing:

    juju deploy prometheus; juju relate prometheus:grafana-source grafana

And again validating the data sources.