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);
It is true if there are many places where this text constant is used. But I see one place only without future extending.
Also this problem is successfully resolved by compiler defines (`#define CV_PARAMETER1 "OPENCV_PARAMETER_1"`), without introducing new enumeration types with dynamic set of values.