Fix launch args for gazebo launch file

[CullenSUN]

Champ_gazebo package has contact_sensor to publish feet contacts with ground in simulation. The issue was champ_gazebo/launch/gazebo.launch.py used a hard coded links map file.

This fix is to use a launch argument so that we can pass in from outside launch files.

After fix:

https://user-images.githubusercontent.com/2050002/230141611-1464ad97-cb82-4dfb-b9e9-28d6b1ec4156.mp4