Update automatic_upgrades to support RHEL/Centos.

Updated to use the new packagekit::cron module for RHEL.

This should provide us automatic package upgrades on our Centos
jenkins slaves.

Fixes LP Bug #1180948.

Change-Id: Iaa67a3573e89339a81761314b662041836f2d3f2
Reviewed-on: https://review.openstack.org/29423
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins