fix creating new shards on rails 3

fixes CNVS-14789

switchman does the whole shebang in a transaction, so slave
won't even have the tables yet

Change-Id: I009b0602647a34f1d5088bb64627466506bdffe5
Reviewed-on: https://gerrit.instructure.com/39310
Reviewed-by: Brian Palmer <brianp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>