fix Shard.in_region scope for non-region-tagged-default-db

Change-Id: Ia1e7c97ac2b5f06f2c24feba9fa42302e659821b
Reviewed-on: https://gerrit.instructure.com/43238
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>