Implement hosting of configuration in a separate service

[jaraco]

Currently, the management of configuration variables happens in a tightly-coupled way inside the VR interface. I imagine this functionality could be decoupled through the following steps:

• Identify or build a standalone service which serves many of the same use cases as VR does (below)
• Update VR UI to reference config in an external service, probably just one URL for environment and another for the settings file.
• VR would still materialize the config at release time and store the materialized config with the release object.

Use cases

• Config is defined in YAML and validated for soundness.
• Allow groups of configuration to be created as a named "ingredient".
• Allow these ingredients to be combined and then overridden, forming a config recipe.
• Config can be either for creating a settings.yaml or for defining environment variables.
• Provide a UI for editing ingredients, assembling recipes, and viewing the rendered config.

Other thoughts

Such a service would be somewhat more complex to deploy as an additional service would need to be installed. However, I can imagine some benefits coming from this decoupling:

• Services other than VR could use this service. It could enable those services to migrate to a VR-backed deployment or enable VR deployments to migrate to another system still relying on the same config.
• Additional features could be added to this service, such as password/secret injection.