[backtrack_behaviour] Don't backtrack if no head camera, smaller pan

This pull:

* Sets the pan to -175 instead of 179 to avoid hitting the limit and cable stretching
* Returns failure if the head camera is not activated in the move_base configuration
* Increases the PTU speed from 20 to 30 by popular demand

Can someone like @lucasb-eyer test if this new limit avoids hitting the hardware limit of the PTU?