Add architecture validation to manifest lists that contain schema 1 manifests

Fixes https://jira.coreos.com/browse/QUAY-1266

workers/tagbackfillworker.py

@@ -72,6 +72,9 @@ class BrokenManifest(ManifestInterface):
   def local_blob_digests(self):
     return []
 
+  def validate(self, content_retriever):
+    pass
+
   def child_manifests(self, lookup_manifest_fn):
     return None