Checking deleting task_state in cloudcafe

Many of our prod tests fail because nova doesn't completely delete servers within 15 mins even though otter issued delete. Otter has recently started checking if `OS-EXT-STS:task_state` is `deleting` after deleting server. If so, it assumes that nova will eventually delete it. This PR does the same in tests. This should reduce tests that fail with following message: `"Servers on the tenant with name test_sg_bhv_srv410275 were not deleted even after waiting 15 mins"`
I am sorry I don't understand how to refactor them? I was thinking of having above two in one line: 
```python
servers = filter(lambda s: s.task_state != 'deleting', 
                        self.server_client.list_servers_with_detail().entity)
```