use default plugin settings for migration context validation

fixes #CNVS-13332

Change-Id: Iab94158b0854cc1a4bdcaf11acc910fce3ed959e
Reviewed-on: https://gerrit.instructure.com/35562
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>