Test to check if the activation-key's content-host limit actually limits its usage

If `server_name` is a class variable, then it is evaluated when the module is imported. `conf.properties['main.server.hostname']` is unset by default. Therefore, `server_name` should not be a class variable so as to avoid an import error. Some of @kbidarkar's other suggestions could work out well, though. For example, something like this:

```python
def setUpClass(cls):
    cls.server_name = ...
```