Recognize the use_sim_time parameter

[wayneparrott]

With the recent addition of parameter features TimeSouce can now take advantage of the use_sim_time parameter for enabling/disabling use of simulation time. See https://github.com/ros2/rclpy/blob/master/rclpy/rclpy/time_source.py for reference.

[wayneparrott]

Note- be sure that RateTimerServer's internal node uses a similar Clock/TimeSource as it's parent (containing) node.