don't inadvertantly commit transaction in migration on mysql

refs CNVS-7032

Change-Id: Idc90c448b538dba9c8cfdbc077bd0809250c9ef6
Reviewed-on: https://gerrit.instructure.com/28864
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>