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``.
This will work:

    hostname=foo
    echo ${foo}

But this will fail:

    echo ${foo}

In the line of code above, the variable being expanded does not exist.