Fixed load car launch file

When launching a big track the spawn entity node would sometimes throw an error saying the /spawn_entity service is unavailable. This was because gazebo has not finished loading within the default timeout of 5 seconds. Setting this timeout to 30 seconds should significantly reduce the chances of that happening, and there will not be many cases in which this 30 seconds is reached because of gazebo not being launched correctly if you use the launcher or launch the track manually since that should always launch gazebo correctly.