Ignore NotFound exception in metadata_set

To avoid a race condition when updating a deleted stack, this catches the
NotFound exception specifically and ignores it.  All other exceptions are
allowed to be raised.

Closes bug: 1582236

Change-Id: I23ddb71c806c0db1ed7ed4ee4ef831ca51605d65

heat/scaling/cooldown.py

@@ -11,6 +11,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from heat.common.exception import NotFound
 from oslo_utils import timeutils
 import six
 
@@ -64,4 +65,7 @@ class CooldownMixin(object):
             now = timeutils.utcnow().isoformat()
             metadata['cooldown'] = {now: cooldown_reason}
         metadata['scaling_in_progress'] = False
-        self.metadata_set(metadata)
+        try:
+            self.metadata_set(metadata)
+        except NotFound:
+            pass