Export gazebo_model path instead of requiring users to install the models

[tfoote]

In the docs right now http://turtlebot3.robotis.com/en/latest/simulation.html the user is explicitly instructed to install the models. This can easily lead to outdated models lingering around.

Instead the GAZEBO_MODEL_PATH it should be declared in the package.xml like this: https://github.com/osrf/car_demo/blob/master/car_demo/package.xml#L25

Also while looking at this the models should not be in the worlds subdirectory either but in a models subdirectory for clarity.

[routiful]

Hi :)

I apply GAZEBO_MODEL_PATH and change name of subdirectory.
I appreciate your contribution.

Thank you!