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);
Internally, when you call getenv, you can convert enumeration constant to text string. But when you promote this text string to API, you can only catch the bug (a typo) at runtime. By using enumeration constant you get more confidence that you did not misspell the parameter name