add depth argument to configure scan_processing. With this configuration...

... scan works for both depth_regratation false and true

It is related with #128 and it resolved #123

scan topic name configuration topic naming using argument existed in groovy but somehow it disappeared in hydro.

*Groovy*
https://github.com/turtlebot/turtlebot/blob/groovy/turtlebot_bringup/launch/3dsensor.launch#L24-L26
https://github.com/turtlebot/turtlebot/blob/groovy/turtlebot_bringup/launch/3dsensor.launch#L83-L90

Hydro : 
https://github.com/turtlebot/turtlebot/blob/hydro/turtlebot_bringup/launch/3dsensor.launch#L22-L23
https://github.com/turtlebot/turtlebot/blob/hydro/turtlebot_bringup/launch/3dsensor.launch#L59-L73