Refactor test cases after adding new params to create_floatingip.

Change-Id: I1feaed67e46edce85397907528e23c4a5f487c1f
diff --git a/neutron_tempest_plugin/scenario/test_mtu.py b/neutron_tempest_plugin/scenario/test_mtu.py
index dbfde9b..7a9f969 100644
--- a/neutron_tempest_plugin/scenario/test_mtu.py
+++ b/neutron_tempest_plugin/scenario/test_mtu.py
@@ -58,7 +58,7 @@
             constants.SERVER_STATUS_ACTIVE)
         port = self.client.list_ports(
             network_id=net['id'], device_id=server['server']['id'])['ports'][0]
-        fip = self.create_and_associate_floatingip(port['id'])
+        fip = self.create_floatingip(port=port)
         return server, fip
 
     def _get_network_params(self):