merging daverigby's update from https://github.com/daverigby/vagrants/commit/6fb2336c1fdf186c275e29b1037d5896a005473a      Disable iptables on CentOS images

    Previously iptables was just 'stopped' which meant if the VM was
    restarted the firewall would be re-enabled. Change to actually
    disabling it which removes it from runlevel.