Don't use private interface in oslo.service

The _ThreadingEvent class in oslo_service.loopingcall is a private
interface, and backported changes to fix performance issues remove that
class. Since the _ThreadingEvent.wait() function is essentially
synonymous with the LoopingCallBase._sleep() method, substitute the
latter instead. This ensures that the Nova tests will work with both
older and newer versions of oslo.service.

This is a stable-only change because these mocks have already been
replaced with the new oslo_service.fixture.SleepFixture on master by
fded752b16.

Change-Id: Ie4f27622f6158b1123b2e9dd050f90a3dbd84ea6
Related-Bug: #1798774

nova/tests/unit/compute/test_compute_mgr.py

@@ -6390,7 +6390,7 @@ class ComputeManagerBuildInstanceTestCase(test.NoDBTestCase):
                 self.context, mock.sentinel.instance,
                 requested_networks=requested_networks)
 
-    @mock.patch('oslo_service.loopingcall._ThreadingEvent.wait',
+    @mock.patch('oslo_service.loopingcall.LoopingCallBase._sleep',
                 new=mock.Mock())
     @mock.patch('nova.compute.manager.ComputeManager._deallocate_network')
     @mock.patch('nova.compute.manager.LOG.warning')

nova/tests/unit/virt/disk/mount/test_api.py

@@ -29,7 +29,7 @@ MAP_PARTITION = "/dev/mapper/nullp77"
 
 
 # Make RetryDecorator not actually sleep on retries
-@mock.patch('oslo_service.loopingcall._ThreadingEvent.wait', new=mock.Mock())
+@mock.patch('oslo_service.loopingcall.LoopingCallBase._sleep', new=mock.Mock())
 class MountTestCase(test.NoDBTestCase):
     def _test_map_dev(self, partition):
         mount = api.Mount(mock.sentinel.image, mock.sentinel.mount_dir)

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

@@ -35,7 +35,7 @@ if sys.version_info > (3,):
 
 
 # Make RetryDecorator not actually sleep on retries
-@mock.patch('oslo_service.loopingcall._ThreadingEvent.wait', new=mock.Mock())
+@mock.patch('oslo_service.loopingcall.LoopingCallBase._sleep', new=mock.Mock())
 class GuestTestCase(test.NoDBTestCase):
 
     def setUp(self):