archive: don't create schema if it already exists

[drewbanin]

When dbt archive is invoked, dbt will always try to create a schema for each archive using:

create schema if not exists {{ schema }};

Instead, dbt should check if the schema already exists, and skip this step if it does indeed exist. If users invoke dbt archive with a user that lacks create schema permissions, the above statement will fail even if the schema already exists! Relevant code is here: https://github.com/fishtown-analytics/dbt/blob/development/dbt/include/global_project/macros/materializations/archive/archive.sql#L113

[drewbanin]

Kicking this out of the 0.10.2 milestone. create permissions for the dbt user are generally expected by dbt. This would be a good enhancement if someone has the cycles to work on it!