Make hostnames default to using $domain instead of example.com as per the documentation

The README says:

> broker_hostname
> node_hostname
> named_hostname
> activemq_hostname
> datastore_hostname
>
> Default: the root plus the domain, e.g. broker.example.com - except named=ns1.example.com

Previous to this change they were defaulting to e.g. "broker.example.com" even if the domain was set to "otherexample.com".  With this change they default to e.g. "broker.otherexample.com"