Fix deprecation warnings in pkg_resources

Fix the following deprecation warnings.

  PkgResourcesDeprecationWarning:
  <MagicMock name='execute().split().__getitem__().__getitem__()'
  id='140417024565696'> is an invalid version and will not be
  supported in a future release

  DeprecationWarning: Creating a LegacyVersion has been
  deprecated and will be removed in the next major release

Change-Id: I23540114120f6ea52754116cfaaeac35e09543b4
Closes-Bug: 1986428
Signed-off-by: Takashi Natsume <[email protected]>
(cherry picked from commit 76cf6b4)

Author: takanattie

neutron/tests/unit/agent/l3/test_dvr_local_router.py

@@ -307,6 +307,7 @@ def test_floating_forward_rules_no_fip_ns(self):
         self.assertFalse(ri.floating_forward_rules(fip))
 
     def test_floating_forward_rules(self):
+        self.utils_exec.return_value = "iptables v1.6.2 (legacy)"
         router = mock.MagicMock()
         router.get.return_value = [{'host': HOSTNAME},
                                    {'host': mock.sentinel.otherhost}]