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``.
The code block below was already in place so I wanted to ask: are we basically creating an 'alias' for ``firewalld`` so that we can invoke ``iptables`` instead? If this is the case then I may have to update my calls to ``firewalld``...