[CentOS] Simpler firewall instructions

Manual editing of iptables is not required nor recommended. The documentation as-is will not open IPv6 ports and might conflict with other services on the same server. It is better to use lokkit, a command line tool to edit the firewall settings. This will edit both iptables configuration files and also the firewall configuration, so that system-config-firewall is updated accordingly. Added bonus: enabling/disabling the firewall using system-config-firewall will not destroy the settings.