Move Gazebo-related stuff out of the turtlebot stack

[bit-pirate]

I.e. Gazebo information inside the URDFs/Xacros in turtlebot_description.

One idea:
Duplicate the robot urdfs from turtlebot_description and add the turtlebot_gazebo.urdf.xacro include where necessary. That duplication s****, but it would allow a clear separation.