Allow nova vmdk-footer test to pass

Nova originally did not support vmdk-with-footer and the test asserted
that fact. Now we need to fix nova and have this test work with both
fixed and non-fixed code. Since the tests are not split by image
format, we can easily skip in the case where an image is marked as
usable, but fails in a known way, which will allow us to pass the
job with and without a fixed nova.

Change-Id: I71268fa915055300e1cb42e592bc62a1f276d733
diff --git a/tempest/api/image/v2/test_images_formats.py b/tempest/api/image/v2/test_images_formats.py
index a234fa2..45626a6 100644
--- a/tempest/api/image/v2/test_images_formats.py
+++ b/tempest/api/image/v2/test_images_formats.py
@@ -168,8 +168,15 @@
         # https://bugs.launchpad.net/glance/+bug/2073262
         is_broken = 'footer' in self.imgdef['name']
 
-        if self.imgdef['usable'] and not is_broken:
-            server = self._create_server_with_image_def(self.imgdef)
+        if self.imgdef['usable']:
+            try:
+                server = self._create_server_with_image_def(self.imgdef)
+            except exceptions.BuildErrorException:
+                if is_broken:
+                    self.skip('Tolerating failed build with known-broken '
+                              'image format')
+                else:
+                    raise
             self.delete_server(server['id'])
         else:
             self.assertRaises(exceptions.BuildErrorException,