Make service-list output consistent

When libvirt automatic disable/enable services, the disabled
reason will be 'None'. However, the column is Nullable and
make it 'None' let the service-list output inconsistent.
This patch changes the default value to NULL.

Change-Id: I958accb500b4b417d755cff8df8ef7d615b710c0
Closes-Bug: #1401480

nova/tests/unit/virt/libvirt/test_driver.py

@@ -7731,7 +7731,7 @@ def test_service_resume_after_broken_connection(self):
             conn.init_host("wibble")
             conn.get_num_instances()
             self.assertTrue(not service_mock.disabled and
-                            service_mock.disabled_reason is 'None')
+                            service_mock.disabled_reason is None)
 
     def test_immediate_delete(self):
         def fake_lookup_by_name(instance_name):

nova/tests/unit/virt/test_virt_drivers.py

@@ -840,7 +840,7 @@ def test_set_host_enabled_when_auto_disabled(self):
                                return_value=service_mock):
             self.connection._set_host_enabled(True)
             self.assertFalse(service_mock.disabled)
-            self.assertEqual(service_mock.disabled_reason, 'None')
+            self.assertIsNone(service_mock.disabled_reason)
 
     def test_set_host_enabled_when_manually_disabled(self):
         self.mox.UnsetStubs()

nova/virt/libvirt/driver.py

@@ -283,7 +283,7 @@
 # The libvirt driver will prefix any disable reason codes with this string.
 DISABLE_PREFIX = 'AUTO: '
 # Disable reason for the service which was enabled or disabled without reason
-DISABLE_REASON_UNDEFINED = 'None'
+DISABLE_REASON_UNDEFINED = None
 
 # Guest config console string
 CONSOLE = "console=tty0 console=ttyS0"
@@ -509,8 +509,6 @@ def _do_quality_warnings(self):
     def _handle_conn_event(self, enabled, reason):
         LOG.info(_LI("Connection event '%(enabled)d' reason '%(reason)s'"),
                  {'enabled': enabled, 'reason': reason})
-        if reason is None:
-            reason = DISABLE_REASON_UNDEFINED
         self._set_host_enabled(enabled, reason)
 
     def init_host(self, host):