[implementation] define project-wide vars in vars.yml, outside of dbt_project.yml

[graciegoheen]

Short description

From #2955

"I want to define project-wide variables outside of my dbt_project.yml."
"I want to be able to reference variables in my dbt_project.yml at parse-time."

Acceptance criteria

• I can define project-wide ("global variables") variables in a vars.yml file, separate from my dbt_project.yml

# vars.yml

vars:
  start_date: '2016-06-01'

• dbt parses vars.yml before parsing dbt_project.yml, meaning default values of vars called in dbt_project.yml would work the way folks expect, rather than how it is today (equivalent to how this works when defining vars through the CLI a la --vars)

Option 1 - behavior change

• You cannot define variables in both places, you must chose the "new" or "legacy" way
• As such, this would need to be behind a behavior change flag
• Open question: Given that behavior change flags are set in dbt_project.yml, would this even be possible?

Option 2 - backwards compatible

• You can define variables in either places
• Open question: Would we still get the parsing benefit if we went this route?

Current workaround: Have a really massive dbt_project.yml file with no ability to reference variables in your project configurations

[b-per]

Is there a reason to make it specific for vars instead of making it possible for any configuration like in this other issue?

[graciegoheen]

@b-per Doing this specifically for vars gives us this added benefit - "I want to be able to reference variables in my dbt_project.yml at parse-time" - because we can then parse the vars.yml before dbt_project.yml

That being said, I don't think this is an either/or - we could do YAML snippets as well :)