Feature request: Config inheritance

ShipIt currently implements replacement config files on a per-environment basis. This is useful when configuration is significantly different between environments.

When configuration differences are limited between environments, this can result in significant duplication. It would be useful for the more specific files to be able to contain an inheritance directive to reduce this duplication. This might look something like this:

shipit.production.yml:

inherit_from: shipit.yml

machine:
  environment:
    PUBLIC: true

shipit.yml:

machine:
  environment:
    PUBLIC: false

deploy:
  override:
    - ./some_deployment_process.sh ${PUBLIC}

...more shared stuff...

We'd be willing to contribute this upstream if it has a reasonable change of being merged. I'm looking for in-principle approval of a feature like this from a maintainer.

Start with app/models/shipit/deploy_spec/file_system.rb, especially the replacement-config handling linked in the issue, and read the configuration examples in README.md. Determine how an environment-specific file could inherit shared values and how conflicts or nested configuration should behave. Done means the inheritance semantics are agreed with maintainers and documented or tested end to end.