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``.
Right. We're not using `format` here, and the entire string is interpreted in the client's shell. When the client interprets this line of code, it will attempt to expand that variable. The variable does not exist on the client. (Does it?)