Don't log 'Not Found' ERROR during image cleanup

It is expected that an image may have already been deleted.

Change-Id: I09db34576e93b3cc090cb85e35678939cf3af82f
diff --git a/tempest/api/compute/base.py b/tempest/api/compute/base.py
index d2f437b..0b527d9 100644
--- a/tempest/api/compute/base.py
+++ b/tempest/api/compute/base.py
@@ -22,6 +22,7 @@
 from tempest.common import isolated_creds
 from tempest.common.utils.data_utils import parse_image_id
 from tempest.common.utils.data_utils import rand_name
+from tempest import exceptions
 from tempest.openstack.common import log as logging
 import tempest.test
 
@@ -103,6 +104,9 @@
         for image_id in cls.images:
             try:
                 cls.images_client.delete_image(image_id)
+            except exceptions.NotFound:
+                # The image may have already been deleted which is OK.
+                pass
             except Exception as exc:
                 LOG.info('Exception raised deleting image %s', image_id)
                 LOG.exception(exc)