Rethink rendering of fields in dbt_project.yml and profiles.yml

[cmcarthur]

Feature

Feature description

dbt is not very explicit about which fields in config files support jinja, and which jinja functions they support. We can do a better job of breaking this up into groups, for example:

• essentially all fields in dbt_project.yml and profiles.yml should support {{env_var(..)}} and {{var(..)}} and should be rendered immediately
• specific fields in dbt_project.yml support a larger context, for example on-run-end accepts a fuller context

Who will this benefit?

everyone

[cmcarthur]

depends on #953

[beckjake]

implementation note: rendering of all hooks really has to wait for runtime. I think all other fields can and should be rendered immediately, though some (like models/seeds/archive) will be tricky since they're pretty free-form.