Test:Check if multiple Activation keys can be attached to a system

- added a function `fetch_associated_content_host` to fetch the associated content-host  from selected activation-key
- updated top menu locator names
This block of code be flattened by doing this:

```python
if not activation_key:
    raise UINoSuchElementError(...)
activation_key.click()
self.wait_for_ajax()
...
```

Also, please use `format`.