Toolkit: make JSON the default output (and use it for "deploy")

[eladb]

We have been seeing some issues around the YAML library we've been using to produce the synthesized output (e.g. #1309, #1100, #876). We are working wit the maintainer to improve the library. However, there is no real reason to use YAML as a wire format (and a default) for the toolkit because mostly it's consumed by machines and not humans.

To avoid further grief, let's move our default to use JSON instead of YAML. We can leave YAML as an option.

[eladb]

This is done.