Added model.config and dummy.urdf for gzweb.

Gzewb needs model.config and GAZEBO_MODEL_PATH to that directory.
By this modification, gzweb should work but actually is not working.
After this PR is merged, I'll ask in https://bitbucket.org/osrf/gzweb.

atlas is displayed gzweb correctly.
cf) atlas_description: https://bitbucket.org/osrf/drcsim/src/4f4be87fd5858e3b72bf05bc684e796d9d452673/atlas_description/?at=default