Fix "NotFound" error in _clear_stacks()

_clear_stacks() will raise "NotFound" exception if stack was
deleted before wait its status. This patch adds a try block
to fix the problem.

Change-Id: I29d4e443e5f355dbba188ccebe930f9401f3ccea
Closes-Bug: #1355666
diff --git a/tempest/api/orchestration/base.py b/tempest/api/orchestration/base.py
index 531df2d..f83169f 100644
--- a/tempest/api/orchestration/base.py
+++ b/tempest/api/orchestration/base.py
@@ -85,8 +85,11 @@
                 pass
 
         for stack_identifier in cls.stacks:
-            cls.client.wait_for_stack_status(
-                stack_identifier, 'DELETE_COMPLETE')
+            try:
+                cls.client.wait_for_stack_status(
+                    stack_identifier, 'DELETE_COMPLETE')
+            except exceptions.NotFound:
+                pass
 
     @classmethod
     def _create_keypair(cls, name_start='keypair-heat-'):