Workaround for different floating_ip_pool results

In one cases result of clients.nova().floating_ip_pools.list()
is list of strings, in other cases it is list of dicts.

Related-Bug: #1355855

Change-Id: I39867702d13aaa9014440ce8d074852af9779547

rally/benchmark/validation.py

@@ -286,6 +286,10 @@ def external_network_exists_validator(config, clients, task):
 
         networks = [net.name for net in
                     clients.nova().floating_ip_pools.list()]
+
+        if isinstance(networks[0], dict):
+            networks = [n["name"] for n in networks]
+
         if ext_network not in networks:
             message = _("External (floating) network with name %(network)s "
                         "not found. "