This fixes the IDs of multi UAVs started with ROS/Gazebo.
Previously the 3 vehicles were displayed in QGC as Vehicle 2, 3, 4.
With this change it is more intuitive Vehicle 1, 2, 3 and this is
also consistent with the way it is documented and how it is in
jMAVSim.
* remove non-functional gazebo headless arg
* remove unused namespace args
* simplify mavros launch, use mavros's px4.launch instead
* fix single_vehicle_spawn spelling
* formatting with xmllint: remove empty lines, set intent to 4 spaces, add xml tag for editors, add comments to break up spections
* remove old and deprecated launch files
Fabian Schilling
Anthony Lamping
Julian Oes
stmoon
TSC21
Kunal Shah
Beat Küng
Mohamed Abdelkader Zahana