fixing cap_pvpapi interface

This PR tries to fix the following 3 reported issues
- http://code.opencv.org/issues/3946
- http://code.opencv.org/issues/3947
- http://code.opencv.org/issues/3948

Basically it enables use of monochrome AVT cameras AND lets the initialize wait long enough since newer cams have a longer initialization time!
This if sentence will not be needed once findNewCamera is used as it already opens the cameras one by one.