Fix rqt_controller_manager for non-humble

[christophfroehlich]

I made a mistake with #1429: rqt_cm does not work on rolling now (sorry): The finally part is always executed, except should be the right python syntax.

Traceback (most recent call last):
  File "/workspaces/ros2_rolling_ws/build/rqt_controller_manager/rqt_controller_manager/controller_manager.py", line 150, in _on_cm_change
    self._update_controllers()
  File "/workspaces/ros2_rolling_ws/build/rqt_controller_manager/rqt_controller_manager/controller_manager.py", line 157, in _update_controllers
    controllers = self._list_controllers()
  File "/workspaces/ros2_rolling_ws/build/rqt_controller_manager/rqt_controller_manager/controller_manager.py", line 181, in _list_controllers
    for name in _get_parameter_controller_names(self._node, self._cm_name):
  File "/workspaces/ros2_rolling_ws/build/rqt_controller_manager/rqt_controller_manager/controller_manager.py", line 424, in _get_parameter_controller_names
    return [n[: -len(suffix)] for n in parameter_names if n.endswith(suffix)]
TypeError: 'Future' object is not iterable

However, I made the version conditioning now by os.environ.get('ROS_DISTRO') what seems to be cleaner than a try/catch imho.

Tested now on rolling and humble.

[saikishor]

Thanks for the fix

[bmagyar]

Much better indeed, thanks!