Add ability to skip for test_signed_image_upload_boot_snapshot

when environment doesn't have an ability to sing image,
this test will fail, for this case
added test skip condition.

Change-Id: I426e0eb9e94386578d5ee2e756edd128a2dd9f0a
Related-prod: PRODX-362
diff --git a/barbican_tempest_plugin/tests/scenario/test_image_signing.py b/barbican_tempest_plugin/tests/scenario/test_image_signing.py
index 90b6586..1b9a2c8 100644
--- a/barbican_tempest_plugin/tests/scenario/test_image_signing.py
+++ b/barbican_tempest_plugin/tests/scenario/test_image_signing.py
@@ -104,6 +104,9 @@
 
     @decorators.idempotent_id('f0603dfd-8b2c-44e2-8b0f-d65c87aab257')
     @utils.services('compute', 'image')
+    @testtools.skipUnless(
+        CONF.compute_feature_enabled.barbican_integration_enabled,
+        'Barbican integration must be enabled.')
     def test_signed_image_upload_boot_snapshot(self):
         """Test that Glance can snapshot an instance using a signed image.