Add pr2_bringup.launch and control respawning of kinect_head.launch

* add pr2_bringup.launch and include it from `/etc/ros/robot.launch` instead of `package://pr2_bringup/pr2.launch`
* add argument `respawn` to kinect_head.launch (defaults to true)
* disable respawning from `/etc/ros/robot.launch` in the case of restarting kinect