spec: don't run shard :once hooks if sharding not enabled

Change-Id: I4b554dc78e3dbeb697d3dc35e5949176917650ed
Reviewed-on: https://gerrit.instructure.com/37821
Reviewed-by: Jon Jensen <jon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>