Fix SITL run script for model override

[Jaeyoung-Lim]

Problem Description The sitl_run script would fail when you are trying to run one of the default models in PX4.

This adds some logic into the run script

• The script searches for the model file in the GAZEBO_MODEL_PATH
• The script looks for the same <model>.sdf file specified with the -m flag. This is the default model PX4 is using for SITL
• When -s is provided this overrides the model name and looks for <sdf model>.sdf file: This allows you to use the same PX4 firmware configs but use different physics / dynamics configured to test (e.g. models in this repo)

Testing For example when you want to run the default PX4 standard_vtol you can run

Tools/sitl_run.sh -m standard_vtol

This will use the standard_vtol.sdf that is located in https://github.com/PX4/PX4-SITL_gazebo/tree/master/models/standard_vtol

If you want to run the standard_vtol with PX4 but with a different model (e.g. standard_vtol_aerodynamics in this repo)

Tools/sitl_run.sh -m standard_vtol -s standard_vtol_aerodynamics

Additional Context

• This problem was identified when trying to run SITL with @tstastny
• I believe you were not using this script for your development. Please use it(or fix it if you don't like part of it), so we can identify problems earlier :smile: