Add CM `switch_controller` service timeout as parameter to spawner.py

[christophfroehlich]

This is a workaround for https://github.com/ros-controls/gz_ros2_control/issues/421 and https://github.com/ros-controls/gazebo_ros2_control/issues/380

If the simulation is started in paused mode, one can now configure also the timeout for the switch_controller service to be completed (the simulation has to be unpaused to perform the controller switches).

Also fixes docs after changes to the spawners in https://github.com/ros-controls/ros2_control/pull/1562