Support different publishing configurations to suport "dev / stage / prod" deployment

[jeffgca]

Basically extend the YAML syntax to support different sections for dev, stage and prod environments, mostly to enable something like

fission app publish --stage

[jeffgca]

Reference from the docs:

Eventually, you will be able to publish different versions of your app -- for example, development, testing, production, etc. -- and use custom domains for your app.

https://guide.fission.codes/developers/cli/fission-yaml