adds mysql integration build step for mysql 8
The supported database policy is listed at: https://grafana.com/docs/grafana/latest/setup-grafana/installation/#supported-databases
As stated in the documentation, Grafana will support the versions of our backend database options that are supported by their projects at the time that a Grafana release is issued.
* Devenv: allow dynamically change mysql_tests docker image * Devenv: allow dynamically change postgres_tests docker image