Comment 4 for bug 1224828

I'm thinking there will need to be a toggle or a subcommand to differentiate a full update from a partial one.

Maybe the following:
heat stack-update --patch ...

And the resulting REST call would be a PATCH rather than a PUT:
https://restful-api-design.readthedocs.org/en/latest/methods.html#patch-vs-put

so to delete a parameter, a full PUT would be required, PATCH is just for changing existing values.