mod_pubsub_serverinfo: Fix check for local domain The module was checking whether a host exists, but this may be misleading if our host is loaded before the host that is being checked. This change makes it check the config. This will provide a false positive in the rare case that the host is defined but not activated/enabled. But we don't have an API currently for "is this host configured and enabled?"

This module was meant to help convert the deprecated [XEP-0156] TXT records into JSON format.

```shell
$ prosodyctl mod_auto156 example.com [another.example ...]
{"links":[{"href":"https://xmpp.example.com/bosh","rel":"urn:xmpp:alt-connections:xbosh"}]}
...
```