Adding secret vars in config.json from exernal file

I suggest to be able to add secret information via external file (as in terraform).

For example, something like that in ``config.json`` :

`"environment_variables": {"authorization_token": var.secret_token }`

And for the command line :

`$chalice deploy --var-file=secret.json`

Start by locating the config.json parsing and the deploy command-line entry point. Compare the proposed environment_variables syntax with the --var-file=secret.json example, then define completion as external secret values being loaded during deployment without storing them directly in config.json.

Written by the indexing model from the issue text.