Added test for add-contenthost/removecontenthost for host-collection

please review it
thanks
Should these strings be unicode? I see that many, many other strings are. Something like this?

```python
result = HostCollection.add_content_host({
    u'id': new_host_col['id'],
    u'organization-id': self.org['id'],
    u'content-host-ids': new_system['id']
})
```