joint_limits: use an open-loop policy for velocity staturation

The feedback from the controller is way too slow to be used on an
actual robot. A robot that had 15 rad.s^-2 on each wheel as
an acceleration limit could not even reach 2 rad.s^-2

This is in line with ros_controllers#23
joint_limits: fix test for open-loop velocity saturation
Merge pull request #4 from po1/joint-limit-open-loop

Open loop policy in joint velocity saturation