CLI update allows users to unset fields

Users may want to remove the entire "metadata" object from their token config or a nested environment variable from their config. There is not an easy way to do this using the waiter cli.

One approach is to use minus - sign at the end of the parameter name (e.g. in kubectl annotate, [stackoverflow](https://stackoverflow.com/questions/54973593/how-to-remove-delete-annotation-in-kubernetes)).