Put all models in the Gazebo Model Database

[CSharpRon]

Release Models supplied by this package must be in the Gazebo model database by the time this package gets released.

For development, models can be accessed by overriding an environment variable which is not good practice for a production machine. Having the models on the model database means that this and other packages can use the models without needing to provide them; Gazebo will handle downloading the models.

Reference the Contribution Guidelines on how to add new models to the database