Merge pull request #9705 from AlexeyAB:dnn_darknet_yolo_v2

diff --cc modules/dnn/include/opencv2/dnn/all_layers.hpp
Simple merge

diff --cc modules/dnn/include/opencv2/dnn/dnn.hpp
Simple merge

diff --cc modules/dnn/src/init.cpp
Simple merge

diff --cc modules/dnn/test/test_layers.cpp
Simple merge