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``).

String formatting uses indexes to accommodate older versions of Python.

Also, cleaned up the ``cdn_install`` task by removing its ``subscribe``
and ``install_prerequisites`` calls since these already already handled
by ``product_install``.
Merge pull request #68 from omaciel/rhel-7

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