Is it possible to refer to os level environment variable in config.json

Hi,
Thank you for providing great python-based serverless framework.

this is not a bug report but a question.

I migrated from [serverless framework](https://serverless.com/) recently. In __serverless framework__ I could set environment os level environment variables via specific syntax.

```
create_s3_bucket:
name: ${env:S3_BUCKET_NAME}
```

https://serverless.com/framework/docs/providers/aws/guide/variables/#referencing-environment-variables

Is it possible to set os level environment variable in chalice config.json in similar way?

I will be grateful for any information you can provide.

Start by reading how Chalice loads config.json and handles environment variables. Determine whether the requested operating-system variable reference is supported, and define completion as either documenting the existing configuration syntax or specifying the behavior needed to add it.

Written by the indexing model from the issue text.