Add SSHConfig method returning hostnames defined in SSH config

For my application, I need to distinguish between explicit host configuration entries and configuration entries synthesized from wildcard entries (e.g. '*'), something which is not possibly using SSHConfig.lookup().

I figure I might not be the only one, hence the pull request. Unit test is included.

Best regards,
Søren Løvborg
Could be written as such with a set comprehension:
```python
return {entry['host'] for entry in self._config}
```