Updated OpenVPN Configuration Guide for CentOS 7

@movesmyers @displague re: [Secure Communications with OpenVPN on CentOS 6](https://www.linode.com/docs/networking/vpn/secure-communications-with-openvpn-on-centos-6)

After spending a day getting OpenVPN running on CentOS 7, I've provided an upgraded version of the CentOS 6 guide for users who wish to do the same. Here is a summary of my major changes.

Best practice for enabling EPEL is to install it through yum itself, not directly with an RPM:

    yum instal epel-release

As of CentOS 7, there have been changes to the networking subsystem and other core functionality which should be noted.

First, iptables has been replaced by [firewalld](https://fedoraproject.org/wiki/FirewallD), so old-style iptables configuration is not supported "out of the box", rendering the suggested iptables configuration impossible. This can be remedied by installing iptables-services.

Also, https://www.linode.com/docs/networking/vpn/secure-communications-with-openvpn-on-centos-6#connect-to-the-openvpn is no longer accurate, and would lead even a reasonably experienced user to frustration as openvpn no longer uses init.d scripts. Any attempt to find documentation for this problem online will lead to frustration, and any attempt to use a previous version of an init.d script for openvpn will also fail due to changes in the networking subsystem. CentOS 7 now uses systemctl, which requires a slightly different startup command, as documented in the ArchLinux wiki: https://wiki.archlinux.org/index.php/OpenVPN#systemd_service_configuration

TODOs:

- Update iptables configuration to reflect the recommended configuration described in https://www.linode.com/docs/security/securing-your-server. Currently there will be conflicts with ports and protocols being open, as well as NAT routing VPN traffic out of the system. This isn't terrible if you're on a new install and don't mind flushing your iptables rules and trying again a few times with rules from other guides from the internet, but it could be problematic for users on established systems.

- Update iptables configuration to use now-current, recommended firewalld.