allow parameterization of timeout for undercarriage_ctrl and velcoity_smoother

This hopefully solves the problem experienced with cob4.

I tried to reproduce the same behaviour on cob3-3. It is not really clear to me, what causes it.

What I found is that it needs several things to be fulfilled:
1. no footprint_observer running
2. you need to press both the deadman-button as well as the "upper_neck" button for bypassing the emergency stop. Then, you need to let go of the upper_neck button and shortly after that (actually almost at the same time)  let go of the deadman-button. Then, no zero velocities are sent and thus the base drives until the watchdogs recognize this and stop the robot

This Pull-Request now lets you specify the time after which the watchdogs stop the robot.