Ros2 - fixing paths to download and find models

[jmguerreroh]

Issue #, if available:

1. Error in the readme file for launcher example
2. Fuel models are downloaded in a folder but .dae files try to find them in the model folder
3. Wrong camera position at starting
4. GAZEBO_MODEL_PATH is wrong, exports sometime fail and it is fixed using the gazebo setup.bash

Description of changes:

1. Fixed readme
2. Changed CMakelist.txt and script to download models in the same folder
3. Changed camera position
4. Added a note to the readme file

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