Launch file may start wrong ign-gazebo version

[chapulina]

ign_gazebo.launch starts Ignition Gazebo with the ign gazebo command line command:

https://github.com/ignitionrobotics/ros_ign/blob/a4bc25fde117f9ce7279c83dc419bfb0eb3983b9/ros_ign_gazebo/launch/ign_gazebo.launch.py#L34

https://github.com/ignitionrobotics/ros_ign/blob/968c31586bbe9964a3788d5970a90f88dfc3cb25/ros_ign_gazebo/scripts/ign_gazebo#L16

The problem is that, if a user has more than one ign-gazebo version installed, that will launch the latest version, which may not necessarily be what the ROS user wants. For example, if I have ros-foxy-ros-ign (which uses Citadel, i.e. ign-gazebo3) and Ignition Dome (i.e. ign-gazebo4) installed, the launch file will start a version of Ignition that may not be compatible with the rest of the things being launched.

The command line tool accepts the --force-version argument to specify what version to launch. But that is quite strict at the moment, and requires passing the fully qualified version, including minor and patch numbers (see gazebosim/gz-tools#36). This makes it complicated to hardcode the version on the launch file.

[caioaamaral]

@chapulina I'm addressing --force-version here at gazebosim/gz-tools#44 and currently writing a bugfix for this one.

[chapulina]

Thanks, @caioaamaral !

[ahcorde]

Added a new launch argument #226