Added test for activation key auto_attach

Added a test that updates an activation key's auto_attach field with the
inverse of its current value.
Also added a field to the ActivationKey entity for auto_attach.
```
@Test: Create an activation key, then update the auto_attach ... ok
```
Actually, I see that you use `ActivationKey.read_json` a bit further down. In that case, it may be better to create an object and use it several times.

```python
activation_key = entities.ActivationKey(id=attrs['id'])
...
response = client.put(
    activation_key.path(),
    ...
)
...
self.assertEqual(
    activation_key.read_json()['auto_attach'],
    not attrs['auto_attach'],
)
```