usb_cam_test: explicitly configure contrast and saturation

In commit bosch-ros-pkg/usb_cam@b25c2938, contrast and saturation
parameters were added to usb_cam. Unfortunately these were hard-coded to
somewhat strange values. Both were turned down towards the bottom of the
range explaining the low-contrast, low-saturation images.

In the same commit, the default focus was set at a very near value
leading to blurry images.

The combination of these factors lead to the appearance of a very broken
webcam indeed.

Set the contrast, saturation and focus explicitly in the launch file and
add a comment explaining their presence. Launch files which use the
usb_cam node should probably explicitly set contrast and saturation from
now on.