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

Updated the testing bundle to:

 juju deploy cs:~rharding/gypsy-danger-testing-2

This includes the changes that add the hook to properly remove the datasource when the relation is broken so that it can be readded a second time.

Tested with mysql and prometheus and verified the datasources are removed and re-add successfully.

This goes with the charm changes in:

https://code.launchpad.net/~rharding/grafana-charm/+git/grafana-charm/+merge/329855