Allow extra data in artifact schema validation

The schema validator for zuul.artifacts returned via zuul_return
did not allow for extra return data (outside of the zuul hierarchy).

Correct the validator to allow that, and update the test to exercise
it.

Change-Id: Ia8d7ed212b03d978bec1a7de1a2647b0c87b7cf9

tests/fixtures/config/sql-driver/git/common-config/playbooks/project-test1.yaml

@@ -3,6 +3,8 @@
     - name: Return artifact data
       zuul_return:
         data:
+          something_else:
+            foo: bar
           zuul:
             artifacts:
               - name: relative

zuul/lib/artifacts.py

@@ -26,7 +26,8 @@ zuul_data = {
         'log_url': str,
         'artifacts': [artifact],
         v.Extra: object,
-    }
+    },
+    v.Extra: object,
 }
 
 artifact_schema = v.Schema(zuul_data)