Add CloudFormation utility functions

This adds some general-purpose functions to create or update an AWS CloudFormation stack from a JSON template file.
I was dimly aware of the `_`-private convention, but nice to know it for sure now, thanks.

I was trying to exit with a non-zero exit code without printing a stack trace, which I think adds little value and distracts from the message. I read somewhere that `sys.exit()` does that, but `os._exit()` doesn't, but now that I actually try it, that seems not to be the case. OK, I'll change that, thanks.