Updated tasks to use ``systemctl`` for RHEL 7.

Updated tasks to use ``systemctl`` instead of ``service`` to manage
services as well as use OS-version specific service names (e.g.
``firewalld`` versus ``iptables``).

Also, cleaned up the ``cdn_install`` task by removing its ``subscribe``
and ``install_prerequisites`` calls since these already already handled
by ``product_install``.
> I've been told that `system enable/disable` automatically starts/stops services explicitly.

Can you say more on this? `systemctl enable foo` does not start foo. If foo is not running and `systemctl enable foo` is executed, `foo` will not be started.