Enforce JSON formatting for arguments.

[jrconlin]

The python code may have been lenient about accepting not strictly formattted JSON arguments. We should enforce JSON compliance with compound args like, Auth Keys, Bridge configuration arguments, etc.

We should ensure that all example files, documentation, etc. also indicates only JSON formatted coding.

(follows up to https://github.com/mozilla-services/autopush-rs/pull/235#pullrequestreview-533735017)