Adding the missing HostDomain config option

The ``HostDomain`` config type have been added few months ago [1]
however the config option have been forgotten and this new type
isn't importable.

When we try to import this type without defining a new related cfg
option we get the following issue:

```
AttributeError: module 'oslo_config.cfg' has no attribute 'HostDomain'
```

These changes allow us to import this new type and allow us to use
it in our configs:

```
>>> from oslo_config import cfg
>>> foo = cfg.HostDomain('foo')
>>> foo.type.__call__("1")
...
During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "~/oslo.config/oslo_config/types.py", line 893, in __call__
     raise ValueError(
ValueError: 1 is not a valid host address
>>> foo.type.__call__("host_name")
'host_name'
```

Also properly initialize HostDomain because The HostDomain class wasn't
calling super in it's __init__() method, which resulted in the type_name not
being set properly for instances of that class.

[1] https://opendev.org/openstack/oslo.config/commit/6480356928c9ae6169ea1e5a5b5f1df3d6e0dc75

Change-Id: Ie947803f61ba0ef080018e0447de894a400d7975
Closes-Bug: 1924283