[Feature] refactor to make sure relationdb is mariadb, to make clear the assumption in the code

[DavyLandman]

Is your feature request related to a problem? Please describe.
As we discussed during the Athens meeting, we should not assume a ML relationaldb is always a mariadb, similar for a documentdb. It is the responsibility of DL to map it to specific backends. We do not currently have to support different backends, but we should not hard code it.

Describe the solution you'd like
In our code we also look at the DL model and use it to check what kind of backend we are running on. It is perfectly fine to only support mariadb under the relationaldb option, but the check should be there in the code, such that it remains a clear extension point.