Changed rviz plugin action server wait to non-simulated time

When using MoveIt! with slowed down simulated time (for me, using ``rosbag play -r 0.1``), the MoveIt Rviz Plugin takes a very long time to load because it waits for the action servers to connect in simulated time. This PR changes the ``wait_for_server`` time to ``WallTime``, which is the actual system/cpu time. Does not change functionality for non-simulated systems, and it simple allows the GUI to load in real time for simulated systems.