dev/guion-bluford regression: allow ref's in dbt_project.yml var configs

[drewbanin]

Issue

Issue description

The vars config in a models: block of dbt_project.yml should support ref() functions, but they currently fail with the error: 'ref' is undefined. I think this indicates that the compilation context for these vars configs has changed, though I'm not sure exactly in which branch that occurred.

In addition to the ref function working, we sure be sure to maintain parity with the context provided in 0.11.x, unless we explicitly choose to make breaking changes.

The error message here is pretty sparse -- it took me a while to track down the source of this issue. @beckjake - are you separately working on related code here? Something about error handling for jinja code sounds familiar, but I might be misremembering things. The actual error reads:

$ dbt -d run
Encountered an error:
Compilation Error
  'ref' is undefined

with no further info about it in the logs.

Results

'ref' is undefined (see example below)

System information

The output of dbt --version: dev/guion-bluford

Steps to reproduce

# dbt_project.yml

models:
    vars:
        abc: '{{ ref("some_model") }}'

$ dbt compile

[beckjake]

vars + project.yml + new bug = Almost certainly caused by PR #1033