Set non-empty db2 driver artifact group

This is to avoid POM-related errors when running gradle `install` task like following one:

```
:dbfit-java:db2:install FAILED
 
FAILURE: Build failed with an exception.
 
* What went wrong:
Execution failed for task ':dbfit-java:db2:install'.
> Could not publish configuration 'archives'
> Unable to initialize POM pom-default.xml: Failed to validate POM for project com.neuri.dbfit:dbfit-db2 at /var/dbfit2/dbfit-java/db2/build/poms/pom-default.xml
```