Avoid false positives with test_server_connectivity_live_migration

The test has been modified to check VM status and connectivity before
obtaining the measured network downtime.
With this change, the network downtime is measured after network
connectivity is recovered after the live-migration is completed,
avoiding false positive results.

Change-Id: I855534b2662d015715d9acd1e4b98614589df6db

tempest/scenario/test_network_advanced_server_ops.py

@@ -270,6 +270,11 @@ class TestNetworkAdvancedServerOps(manager.NetworkScenarioTest):
         new_host = self.get_host_for_server(server['id'])
         self.assertNotEqual(old_host, new_host, 'Server did not migrate')
 
+        # we first wait until the VM replies pings again, then check the
+        # network downtime
+        self._wait_server_status_and_check_network_connectivity(
+            server, keypair, floating_ip)
+
         downtime = downtime_meter.get_downtime()
         self.assertIsNotNone(downtime)
         LOG.debug("Downtime seconds measured with downtime_meter = %r",
@@ -280,9 +285,6 @@ class TestNetworkAdvancedServerOps(manager.NetworkScenarioTest):
             "Downtime of {} seconds is higher than expected '{}'".format(
                 downtime, allowed_downtime))
 
-        self._wait_server_status_and_check_network_connectivity(
-            server, keypair, floating_ip)
-
     @decorators.idempotent_id('25b188d7-0183-4b1e-a11d-15840c8e2fd6')
     @testtools.skipUnless(CONF.compute_feature_enabled.cold_migration,
                           'Cold migration is not available.')