Add multi-agent launch file

[TannerGilbert]

I would like to add a multi-agent launch file similar to what was already discussed initially in ArduPilot/ardupilot_gazebo#89. I already have a working multiagent simulation that I use for a project I'm involved in and I think this can be adapted well as a general example especially after #64 is merged which adds spawning of the models via ros_gz_sim create instead of directly putting the drone model into the world file.

Is there any interest in having this?