[generator] add a choices var to generator.py

[fearful-symmetry]

See #12765 for the bug

Adding a choices var to the parser argument seemed like the most elegant and most pythonic way to fix this. We might not want to add error handling to os.walk, since presumably it ignores all errors for a reason. I also imagine this script won't last too much longer, as we need to migrate all this to mage anyway.