Fix for drop statement which was not namespaced to a schema

[drewbanin]

The backup_relation was previously created without a schema. This results in a (correct) rename query which looked like:

alter table schema.table rename to table__dbt_backup_20191118

The corresponding drop however also looked like:

drop table if exists table__dbt_backup_20191118

This was incorrect, as it this query is looking for a table called table__dbt_backup_20191118 in the public schema! As a result, full refresh builds on postgres + redshift may have been leaving backup tables in dbt schemas since at least 0.14.3, though possibly earlier.

[beckjake]

lgtm!