Fix: add section to troubleshoot Avahi and mDNS (#771)
Fix: add section to troubleshoot Avahi and mDNS (#771)

* Docs: add mdns.allow to troubleshooting section

I was resolution problems with Avahi on an openSUSE 15.2 host trying to
resolve a sumaform'ed Uyuni server:

```
opensuse # avahi-resolve -n uyuni.tf.local
uyuni.tf.local 192.168.122.154
ping: uyuni.tf.local: Name or service not known
PING 192.168.122.154 (192.168.122.154) 56(84) bytes of data.
64 bytes from 192.168.122.154: icmp_seq=1 ttl=64 time=0.245 ms
```

The problem disappears when I change Avahi configuration in the Uyuni
host to use the first-level domain `local` (instead of `tf.local`).

Turns out that in the newest version of `nss-mdns` (shipping with
OpenSUSE 15.2), second-level `.local` domains must be explicitly allowed
to be resolved in `/etc/mdns.allow` AND `mdns` (not `mdns_minimal`) must
be used in `nsswitch.conf`.
The latter is already covered in the troubleshooting, I took care of
adding the former information into the docs.

Upstream issue: https://github.com/lathiat/nss-mdns/issues/64
Upstream resolution (linked in sumaform docs):
https://github.com/lathiat/nss-mdns/blob/master/README.md#etcmdnsallow
moio/sumaform,moio/sumaform,moio/sumaform