Support configOverrides #756
Description
Currently users can not configOverride the generated config.json.
This is a real bummer, as this prohibits any customization by the user.
However, this is not trivially done, as it's hard to map a BTreeMap<String, String> to arbitrary JSON (compared to Hadoop xml or many .properties files).
Note: This is a hard problem we have for other products as well: E.g. there is no way to configure something in the authentication and authorization XML of NiFI. Also likely other files, basically where it is not a simple key/value config. We should try to come up with a generic and consistent way for all arbitrary config formats.