Dependency on python-opencv-headless

[DXist]

There is no need for GUI visualization functionality in server environments. In such environments it's possible to use headless variant of opencv (python-opencv-headless) which doesn't require extra dependencies (https://github.com/open-mmlab/mmdetection/blob/df1b90430444b19b263082337c2a02ea0369dc1e/docker/Dockerfile#L7)

[hellock]

#146