do not modify parameters unless explicitly set

The contrast, saturation, brightness, sharpness and focus parameters were recently added to usb_cam in b25c293. This caused a regression for us (sigproc/robotic_surgery#17) whereby, unles reset explicitly, the default settings for a webcam are overridden in all cases by the hard-coded defaults in usb_cam. This caused a previously working setup to start behaving very strangely.

It's probably near impossible to find a set of "known good" defaults for all cameras and so this PR changes the behaviour of the node to simply avoid setting the contrast, saturation, etc unless they've been explicitly set in the launch file. This is an application of the "principle of least surprise".