Add support for env vars in playbook

[colmsnowplow]

We added support for env vars here - however it looks like this is evaluated at runtime and can only be used as a template within actual SQL files.

It would be a nice quality of life improvement to be able to set parts of the playbook configuration via env vars too - for example connection details for the database. I'm quite fond of the approach that great_expectations takes to this problem - which allows one to use a separate YAML file for creds, environment variables, or a combination of both.

It brings a much lower risk of leaking credentials.

[colmsnowplow]

Looks like I reported this one already then forgot 🤦 #148