[human_aware_navigation] Reading default velocities from move_base params on start

Fixing #86 

Also not reconfiguring the min values any more. Those are never used but overwrite the set parameters.

The only problem is, that it only reads the parameters on start-up. If they are reconfigured afterwards, it will still reset them to the default ones it got before. Originally, I was reading the default params every time the goal callback was triggered (see line 168) but that led to the error of move_base failing to be reconfigured once in a while causing the human aware navigation to use the already slower values and never recovering to full speed.

Not tested on on robot, @hawesie if you have to restart at some point you might want to try this.