Firewall silently blocking Consul Connect gateways and how to solve it

### Proposal
I've just spent around 3 days figuring out, why my gateways cause traffic to close connections unexpectedly.
I finally tried disabling firewall, restarted all allocations (properly, not in-place) and it started to work.
I tried adding the nomad interface to the firewalls trusted zone and it started to work.

I'd welcome, if either:
Docs would mention somewhere (I haven't found it at least) that it might be needed to run
```bash
# firewall-cmd --zone=trusted --add-interface=nomad
# firewall-cmd --permanent --zone=trusted --add-interface=nomad
```
or the equivalent for OSes without firewall-cmd.

or the nomad itself would, during installation (at least when installing from hashicorp repository), set it up just like docker does it for it's interface `docker0`

### Use-cases
Well, it allowed me to finally use Ingress (and Terminating I guess, but haven't tried that yet) gateways.

### Attempted Solutions
Doing a search for `firewall` using the Nomad's docs search bar
Also looking everywhere (and it was not mentioned once anywhere) to find solution for my problem.

Start by searching the Nomad documentation for firewall, Consul Connect gateways, and the nomad interface. Document the trusted-interface requirement and the firewall-cmd commands shown in the issue, including guidance for equivalent configurations on other operating systems; it is done when users can find this setup information while configuring gateways.

Written by the indexing model from the issue text.