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``.
From `man systemctl`:

    Note that this does not have the effect of also starting any of the units being enabled.

Also, my personal experience with systemd indicates that `enable` does not also start a service. enable/disable affects what happens at boot-time, and start/stop affects the state of the service immediately.