Add optional parameter to set the joystick device

This is useful for
a) Multicopter simulations, or
b) If multiple joysticks are attached to the system and a specific one should be used to control the quadcopter.

It did not work for me to just use "rosparam set joy_node/dev /dev/input/jsX" before launching the hector_quadrotor_teleop (ROS Indigo).

I tested it with the XBOX controller which was attached to /dev/input/js1 and the indoor_slam demo.