OCL: added runtime flags for debugging

Added environment parameters to control execution of OpenCL code:
- `OPENCV_OPENCL_RAISE_ERROR` - call `CV_Assert` to validate success of OpenCL runtime calls (raise on error);
- `OPENCV_OPENCL_PERF_CHECK_BYPASS` - to bypass performance checks and call OpenCL optimization in the most cases (usage example in arithm.cpp & stat.cpp files);
Text strings is a natural way to use with environment variables.
Can you describe this problem?