only clear cache for default shard

so that we don't accidentally clear it for each new shard

Change-Id: I93f760e72646ae0231a8d6659ea3b5370ecbac10
Reviewed-on: https://gerrit.instructure.com/38913
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>