Depth only sensors

Fixed a annoying bug that prevents depth only sensors from properly calculating the point cloud due to incorrect focal length.

It was very hard to trace, but I noticed that the focal length was zero when passed to depth_image_proc/point_cloud_xyz nodelet 

I also had to modify the depth.launch.xml , and use in the depth_image_proc/point_cloud_xyz nodelet  image_raw instead of image_rect_raw (was not being generated due to the lack of calibration file)

I tested this change with:
  - Xtion Pro Live
  - Xtion Pro
  - Occipital Structure sensor

and it worked with all of them !