don't overwrite birth shard if it already exists

refs CNVS-14250

Change-Id: I2e8defd8c27ddad48c581735c85e563f5a1a9946
Reviewed-on: https://gerrit.instructure.com/37721
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>