Should provide option to set shell=False in launch files

[azeey]

Due to ros2/launch#757 Gazebo launched by gz_sim.launch.py does not shut down cleanly. We should provide a way to set shell=False to workaround this issue. Most users probably don't need shell=True, which is mainly used to expand environment variables and file globs [1] since launch provides a way to set environment variables and find absolute paths to files, so we might consider making shell=False, the default.