Fix _get_rate_limit when resp is None

https://review.openstack.org/332848 added retry logic for the client.
Function _get_rate_limit may receive resp=None (seen in the Sahara
unit tests), and in that case we get an exception.

Change-Id: Ibfbb10087121bae7d6f4abdd4cdb8d04d039c970

cinderclient/exceptions.py

@@ -160,7 +160,7 @@ class OverLimit(ClientException):
         self._get_rate_limit(response)
 
     def _get_rate_limit(self, resp):
-        if resp.headers:
+        if (resp is not None) and resp.headers:
             utc_now = timeutils.utcnow()
             value = resp.headers.get('Retry-After', '0')
             try: