Add support for environment variables

[lfvjimisola]

We have experience with dynaconf internally for our yaml files. We use it as a pre-processor for our yaml files before it is passed on to ruamel.

Dynaconf adds support for e.g.:

• Full support for environment variables to override existing settings (dotenv support included).
• Built-in support for Hashicorp Vault and Redis as settings and secrets storage.