Add a way to override the used simulation file or simulationData using environment variables

As multiple plugins can be loaded the system needs to support configuring only a specific backend. To do so the SimulationEngine instance needs to get an identifier which can also be used in the environment variables. By default this identifier can be autogenerated to be the plugin name. E.g. qtivivehiclefunctionsplugin

The environment variables could then look like this:

IVI_SIMULATION_OVERRIDE=<identifier>=<QML-file>
IVI_SIMULATION_DATA_OVERRIDE=<identifier>=<JSON-file>