Option to load and merge several configuration files about bb-storage

[ailiev]

The alternative would be for the deployer to do some kind of templating and merging outside the barn process. This is extra work for many users, and also complicated by having most barn components in a bare Docker image without any tools to do the file munging at runtime. (The bare image is fine in itself I'd say)

[mickael-carl]

One of the reasons of moving all configuration to json based-file is that you can use jsonnet to actually template the common part of each components, rather than adding extra logic to merge config files.
I think using jsonnet is simple enough that it can be done by most users of Buildbarn. However we can also provide examples in the bb-deployment repo to help users get started, and that should solve the usability issue?

[mickael-carl]

@ailiev this is now supported out of the box since a56d3a1: you can feed bb-storage (and all other components of Buildbarn) Jsonnet files and it will render them for you, allowing you to merge multiple configuration files without depending on any extra tooling.
bb-deployments will be updated to reflect that change shortly.