Allow to change the gains/values from analog sticks in the PS4 joystick example code

[ricardodeazambuja]

The analog sticks generate values from 0 to 255 (at least this is what I can get using evdev) , they are normalized (0 to 1) and multiplied by 1000. It's much easier to control the drone if a smaller gain is used instead of 1000 and, ideally, this should be easy to set through command line instead of hard coding it...