Refactor MDSAL Northbound test to use cars/people model

The current MDSAL Northbound test plays with configuration in the urn:opendaylight:params:xml:ns:yang:controller:config. However the purpose of the suite is to check that the MDSAL Northbound feature of ODL works, not that the controller configuration is behaving properly. The suite was supposed to be usable across ODL releases.

The problem with the suite is that now the Lithium and the Beryllium releases have different configs in that namespace and thus two different suites would be needed. And since Boron is most likely going to have another different set of configs in the namespace, it would need yet another test suite. The cars/people model was created precisely to avoid these spurious dependencies.