dbt fails without create schema permissions

[jthandy]

dbt is trying to run the following during dbt run:

cursor.execute('create schema if not exists "{}"'.format(target_cfg['schema']))

this is a convenient way to create the schema if it doesn't exist, but it requires the user to have create schema privileges. it's reasonable to assume the user will not have create schema privileges, and in this case dbt will just need to verify that the requested schema does, in fact, exist--and it will need to do so in a way that does not require admin privileges (aka does not use system tables).