cdp-listend port (7777/udp) blocked by default by firewalld

If you install `firewalld` on a system with the default configuration, cdp-listend notifications received on port `7777/udp` are blocked. Not clear how it should be solved as we don't have a component to manage `firewalld` if I'm right. The command to fix the problem is:

```
firewall-cmd --add-port 7777/udp
```