Stabilize unshelve notification sample tests

The notification sample test for unshelve waits for the instance to
reach ACTIVE state and then asserts if unshelve.end notification is
emitted properly. However the instance.vm_state is set to ACTIVE earlier[1]
than emitting the unshelve.end notification[2]. This can cause two
different test case failure.

1) _test_unshelve_server() fails with no ushelve.end notification
received.

2) _test_shelve_and_shelve_offload_server() also has a unshelve action
at the end and that test step also only waits for the the ACTIVE state.
So the unshelve.end notification from the end of
_test_shelve_and_shelve_offload_server() can bleed into the
_test_unshelve_server() step causing that it receive one more
notifications.

So this patch adds an extra
self._wait_for_notification('instance.unshelve.end') call to each test
step to prevent the instability.

[1] https://github.com/openstack/nova/blob/5c6c1f8f/nova/compute/manager.py#L5322-L5326
[2] https://github.com/openstack/nova/blob/5c6c1f8f/nova/compute/manager.py#L5329
[3] https://github.com/openstack/nova/blob/5c6c1f8f/nova/tests/functional/notification_sample_tests/test_instance.py#L836

Closes-Bug: #1835070

Change-Id: Ie217523a8969326b27930d7f74e50e9b352ab7a1
(cherry picked from commit 40f1e7c)

Author: Balazs Gibizer

nova/tests/functional/notification_sample_tests/test_instance.py

@@ -841,6 +841,7 @@ def _test_shelve_and_shelve_offload_server(self, server):
 
         self.api.post_server_action(server['id'], {'unshelve': None})
         self._wait_for_state_change(self.api, server, 'ACTIVE')
+        self._wait_for_notification('instance.unshelve.end')
 
     def _test_unshelve_server(self, server):
         # setting the shelved_offload_time to 0 should set the
@@ -858,6 +859,7 @@ def _test_unshelve_server(self, server):
         post = {'unshelve': None}
         self.api.post_server_action(server['id'], post)
         self._wait_for_state_change(self.admin_api, server, 'ACTIVE')
+        self._wait_for_notification('instance.unshelve.end')
         self.assertEqual(9, len(fake_notifier.VERSIONED_NOTIFICATIONS))
         self._verify_notification(
             'instance-unshelve-start',