Add support for RHEL to site.pp.

Updates site.pp with a block for RHEL slaves.

Change-Id: Ib83e9d0cd1cc4208eac3a0a63580c76e60965c2a
Reviewed-on: https://review.openstack.org/23106
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins