Add the host extension.

This extension loads in Neutron using the proper api_extensions_path
configuration attribute in *neutron.conf*. Even though is done in the
common Neutron way, we can merge it and @tfukushima can use it as
testing extension.
You might be able to reduce the lines of code using resource_helper, like so:

```python
    @classmethod
    def get_resources(cls):
        plural_mappings = resource_helper.build_plural_mappings(
            {}, RESOURCE_ATTRIBUTE_MAP)
        attr.PLURALS.update(plural_mappings)
        import pdb
        pdb.set_trace()
        return resource_helper.build_resource_info(plural_mappings,
                                                   RESOURCE_ATTRIBUTE_MAP,
                                                   None)
```