Support for env-vars in cmd-line args

If the env-var being added is a secret, its not possible to commit it into the config file. The aws cli tool supports them as `--environment "Variables={K=V}"`. This is great because you can specify these in a build box more securely than a file in the repo.

Start by tracing Chalice's command-line argument handling and how environment variables are read during deployment. Compare the requested syntax with the AWS CLI behavior described in the issue, then verify that a secret value can be supplied without committing it to the configuration file.

Written by the indexing model from the issue text.