Use LOG.warning instead of deprecated LOG.warn

The LOG.warn method is deprecated[1] and the LOG.warning method should
be used instead.

[1] https://docs.python.org/3/library/logging.html#logging.warning

Change-Id: Idfe1b7d7dce1dcf1799865c24ffb2c43f423e3ad
diff --git a/neutron_tempest_plugin/scenario/test_ports.py b/neutron_tempest_plugin/scenario/test_ports.py
index fb2d2b0..c661d39 100644
--- a/neutron_tempest_plugin/scenario/test_ports.py
+++ b/neutron_tempest_plugin/scenario/test_ports.py
@@ -107,8 +107,8 @@
                 if port is not None:
                     break
             except Exception as e:
-                LOG.warn('Failed to create Port, using Fixed_IP:{}, '
-                         'the Error was:{}'.format(ip, e))
+                LOG.warning('Failed to create Port, using Fixed_IP:{}, '
+                            'the Error was:{}'.format(ip, e))
         fip, server = self._create_instance_with_port(port)
         self.check_connectivity(fip[0]['floating_ip_address'],
                                 CONF.validation.image_ssh_user,