Unify the get_layers calls across all implements of manifest schemas to ensure we have a common type returned

Also renames some methods to make it more clear what kind of information they return

test/registry/protocol_v2.py

@@ -543,20 +543,21 @@ class V2Protocol(RegistryProtocol):
       if manifest.schema_version == 1:
         image_ids[tag_name] = manifest.leaf_layer_v1_image_id
 
-      # Verify the layers.
+      # Verify the blobs.
       layer_index = 0
       empty_count = 0
+      blob_digests = list(manifest.blob_digests)
       for image in images:
         if manifest.schema_version == 2 and image.is_empty:
           empty_count += 1
           continue
 
         # If the layer is remote, then we expect the blob to *not* exist in the system.
-        layer = manifest.layers[layer_index]
+        blob_digest = blob_digests[layer_index]
         expected_status = 404 if image.urls else 200
         result = self.conduct(session, 'GET',
                               '/v2/%s/blobs/%s' % (self.repo_name(namespace, repo_name),
-                                                   layer.digest),
+                                                   blob_digest),
                               expected_status=expected_status,
                               headers=headers)
 
@@ -565,7 +566,7 @@ class V2Protocol(RegistryProtocol):
 
         layer_index += 1
 
-      assert (len(manifest.layers) + empty_count) == len(images)
+      assert (len(blob_digests) + empty_count) >= len(images) # Schema 2 has 1 extra for config
 
     return PullResult(manifests=manifests, image_ids=image_ids)