Wait for deletion_time in integration tests

When we check for DELETE_COMPLETE, we can have a window where the stack
is mark as such but without deletion_time. Let's wait for that
information.

Change-Id: Ia8c225aab8b0f118e99963fc22341d743e9182a1
Closes-Bug: #1625661
diff --git a/common/test.py b/common/test.py
index b9bb036..b422012 100644
--- a/common/test.py
+++ b/common/test.py
@@ -285,6 +285,10 @@
                         stack_identifier) != stack.updated_time:
                     self.updated_time[stack_identifier] = stack.updated_time
                     return True
+            elif status == 'DELETE_COMPLETE' and stack.deletion_time is None:
+                # Wait for deleted_time to be filled, so that we have more
+                # confidence the operation is finished.
+                return False
             else:
                 return True