Dropped "cloudServers" endpoint fix

It seems that the "cloudServers" API endpoint (for 1st gen cloudservers) is no longer being returned in the response for new Rackspace Cloud accounts.  This causes cloudlb to fail with the following

```python
>>> import cloudlb
>>> clb = cloudlb.CloudLoadBalancer(username, api_key, 'lon')
>>> clb.loadbalancers.list()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/lemuelf/.virtualenvs/cloudlb-test/src/python-cloudlb/cloudlb/loadbalancers.py", line 127, in list
    self._list("/loadbalancers.json", "loadBalancers") \
  File "/Users/lemuelf/.virtualenvs/cloudlb-test/src/python-cloudlb/cloudlb/base.py", line 59, in _list
    resp, body = self.api.client.get(url)
  File "/Users/lemuelf/.virtualenvs/cloudlb-test/src/python-cloudlb/cloudlb/client.py", line 191, in get
    return self._cloudlb_request(url, 'GET', **kwargs)
  File "/Users/lemuelf/.virtualenvs/cloudlb-test/src/python-cloudlb/cloudlb/client.py", line 87, in _cloudlb_request
    self.authenticate()
  File "/Users/lemuelf/.virtualenvs/cloudlb-test/src/python-cloudlb/cloudlb/client.py", line 79, in authenticate
    auth_data['serviceCatalog']['cloudServers'][0]['publicURL']))
KeyError: 'cloudServers'
```

This pull request changes "cloudServers" endpoint being used to determine account number, with the "cloudServersOpenStack" endpoint (not sure, though, if that's the safest choice).