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``.
I do not understand why `systemctl mask` is used.

```
mask NAME...
           Mask one or more unit files, as specified on the command line. This will link these units to /dev/null, making it impossible to start them. This is a stronger version of
           disable, since it prohibits all kinds of activation of the unit, including enablement and manual activation. Use this option with care. This honors the --runtime option
           to only mask temporarily until the next reboot of the system.
```