Support environment variables in compilation context for profile field

[drewbanin]

Describe the bug

dbt v0.16.0 rationalized the compilation context for fields in the dbt_project.yml file. One key field, profile, is required to bootstrap the target variable which can then be used in other places in the dbt_project.yml compilation context.

It looks like 0.16.0 removed compilation of the profile field altogether. Can we instead compile this field as a jinja string with "base context" described here?

Steps To Reproduce

# dbt_project.yml

profile: "{{ env_var('DBT_PROFILE_NAME') }}"

### Alternatives
- Pass an env var in on the CLI:

dbt run --profile $DBT_PROFILE_NAME

• Add a builtin env var, DBT_PROFILE_NAME that would override the profile if provided

[drewbanin]

Let's use the base context to compile the profile name string