fix Rally task [NovaServers.list_servers] failed with network problems

when we get 'nics' information from rally task config
'contexts', the type of self.config["nics"] is 'tuple', but we need it is 'list'
type.

Change-Id: If293700a5c0a2f30a23b1fcf808adb6e467c76c6
Closes-Bug: #1695245

rally/plugins/openstack/context/nova/servers.py

@@ -91,7 +91,7 @@ class ServerGenerator(context.Context):
         if self.config.get("nics"):
             if isinstance(self.config["nics"][0], dict):
                 # it is a format that Nova API expects
-                kwargs["nics"] = self.config["nics"]
+                kwargs["nics"] = list(self.config["nics"])
             else:
                 kwargs["nics"] = [{"net-id": nic}
                                   for nic in self.config["nics"]]