Winkler/fusion readiness

[matt-winkler]

Description & motivation

resolves: #367

Checklist

• I have verified that these changes work locally
• I have updated the README.md (if applicable)
• I have added an integration test for my fix/feature (if applicable)

[matt-winkler]

@dataders couple things I'm running into:

• Are --vars supported on Fusion? Error message seems to indicate "no"
• dbtf run-operation dbt_external_tables.stage_external_sources is giving an error message indicating the USE DATABASE isn't being passed for step 11, and/or the SQL doesn't have the FQN. I tried a few things modifying the macros in create_snowpipe.sql and get_copy_sql.sql but no impact.