Do not delete server when task state is image_snapshot

test_delete_saving_image fails when deleting server while server
is in task_state image_snapshot, so this is to wait for the server
image-snapshotting to be finished before deleting the server.

Change-Id: I09e0a796ede5509b84bd844488444d77f1763418
Closes-Bug: #1839048
diff --git a/tempest/api/compute/images/test_images.py b/tempest/api/compute/images/test_images.py
index c8221c2..7cf26fb 100644
--- a/tempest/api/compute/images/test_images.py
+++ b/tempest/api/compute/images/test_images.py
@@ -47,6 +47,10 @@
     def test_delete_saving_image(self):
         server = self.create_test_server(wait_until='ACTIVE')
         self.addCleanup(self.servers_client.delete_server, server['id'])
+        # wait for server active to avoid conflict when deleting server
+        # in task_state image_snapshot
+        self.addCleanup(waiters.wait_for_server_status, self.servers_client,
+                        server['id'], 'ACTIVE')
         image = self.create_image_from_server(server['id'],
                                               wait_until='SAVING')
         self.client.delete_image(image['id'])