* Update launch files to support multi-video stream in SITL
* Pass extra args to jinja_gen.py inside gazebo_sitl_multiple_run.sh to enable multi-video streaming
* 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