avoid calling all_models during migrations if possible

Change-Id: Iaff0d334b0a05e0372197d4eb0643289a9408c3f
Reviewed-on: https://gerrit.instructure.com/37743
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>