Fix clustering on multiple columns (BigQuery)

[drewbanin]

Issue

Issue description

dbt 0.11.1 injects SQL wrapped in parens for clustering keys:
https://github.com/fishtown-analytics/dbt/blob/dev/guion-bluford/dbt/include/global_project/macros/adapters/bigquery.sql#L14-L28

When a list of columns are provided in the cluster_by config, this evaluates to a query like:

create or replace table dbt_dbanin.orders
partition by date(created_at)
cluster by (region,order_id)    --- <--- this is incorrect
as (
    select current_timestamp as created_at, 1 as region, 2 as order_id
);

This results in an error like:

CLUSTER BY expression must be groupable, but type is STRUCT

This can be fixed by removing the parens around the cluster by clause, eg:

create or replace table dbt_dbanin.orders
partition by date(created_at)
cluster by region, order_id
...

Results

BigQuery returns the following error:

CLUSTER BY expression must be groupable, but type is STRUCT

System information

dbt 0.11.1