vdk-core: make db_default_type case insensitive

[antoniivanov]

It is a common error where you can set db_default_type=sqlite but because it's declared in the plugin as "SQLITE" we get error that no such db connection type exists which is not true.

This PR changes it so that the db type check is not case sensitive

Testing Done: unit tests

Signed-off-by: Antoni Ivanov [email protected]