correct position of walkers.

[heweweh]

I'm trying to generate training data with carla-simulator, and fetch image/pointcloud/objects information via carla-ros-bridge.
the problem is the message at /carla/ego_vehicle/objects or /carla/objects topic with [derived_object_msgs](https://docs.ros.org/en/kinetic/api/derived_object_msgs/html/index-msg.html)/Object type.

I found the object_sensor is publishing objects like following behaviors:

1. if the object is a vehicle, then the Object.pose.position will be the position on the ground(x-y plane), but it will be adjust to pivot point when publishing Marker.
2. if the object is a walker, then the Object.pose.position will be the pivot point of bounding box, and there is no adjustment before publishing Marker.

the reason, that I consider these behaviors are problematic, is the behaviors are not uniform, so I made this patch.
the reason, that I choose to modify the walker.py, is that the position is intentionally adjusted when publishing Marker in vehicle.py, so I think maintainers are proposing to publish objects topic with position on the ground, but not the pivot point.

please help me to confirm this MR, thanks.

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