Move periodic test times into US Dark (0600 UTC).

Change-Id: Iebc35ee6199ab1d57640cceaa523b770ebea37a7
Reviewed-on: https://review.openstack.org/19347
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Sean Dague <sdague@linux.vnet.ibm.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins