Create DDL drop scripts for supported databases

The current issue is that we create DB schema using DDL scripts but then we drop it by using hibernate metadata. Metadata have changed and may change in future hibernate versions, so names for the foreign keys won't match anymore. Therefore, we should stabilize it by using DDL drop scripts as a counterpart to create scripts.
This will allow us to execute test against DDL scripts even with redeployments, e.g. when testing jbpm-in-container, where DB schema is frequently created and dropped.