create users on master for orphaned attachments migration

Change-Id: I00acce0a950d4f2be2b6dfb439338b8dad1de50d
Reviewed-on: https://gerrit.instructure.com/29957
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>