Various fixes coming out of discussions in #2436 (#2497)
- Add --skip_tests option to build.py based on github feedback
- Add debug output at end of run_subprocess so it's clearer when the output is from a different process running
- Add check for scipy as it's required by gen_test_models.py for the onnx tests
- Use log.warning instead of warnings.warn for consistency. We use the logger almost everywhere and somewhat randomly used warnings.warn in two places.
- Add check for 'wheel' dependency not being found in setup.py and handle more gracefully
- Fix invalid input name in Keras tests
