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"`
My apologies, I thought I replied to this via e-mail, but apparently the comment doesn't show up.

The two lines here have the same behavior as the ones [in the previous hunk](https://github.com/rackerlabs/otter/pull/641/files#diff-de2691e05e7741bbe6184a12a154de4eR258). That behavior should be refactored into a function:

```python
def get_servers(client):
    return [server for server in client.list_servers_with_detail().entity if server.task_state != "deleting"]
```

Except with a better name than `get_servers`. `get_up_servers`? I don't know :)