Added a new 'velocity_scaling_factor' parameter feature to joint_limits.yaml

This evenly reduces max joint velocity for all joints. It is particularly useful if you are using the default max_velocities from the URDF, but want to slow down your robot say by 10% (set it to 0.1)

As we discussed @mmurooka

This PR is documented via the Setup Assistant https://github.com/ros-planning/moveit_setup_assistant/pull/92