More changes for registry-v2 in python.

Implement the minimal changes to the local filesystem storage driver and feed them through the distributed storage driver.
Create a digest package which contains digest_tools and checksums.
Fix the tests to use the new v1 endpoint locations.
Fix repository.delete_instance to properly filter the generated queries to avoid most subquery deletes, but still generate them when not explicitly filtered.

data/model/blob.py

@@ -0,0 +1,22 @@
+from data.model import config, DataModelException
+
+from data.database import ImageStorage, Image, ImageStorageLocation, ImageStoragePlacement
+
+
+class BlobDoesNotExist(DataModelException):
+  pass
+
+
+def get_blob_by_digest(blob_digest):
+  try:
+    return ImageStorage.get(checksum=blob_digest)
+  except ImageStorage.DoesNotExist:
+    raise BlobDoesNotExist('Blob does not exist with digest: {0}'.format(blob_digest))
+
+
+def store_blob_record(blob_digest, location_name):
+  storage = ImageStorage.create(checksum=blob_digest)
+  location = ImageStorageLocation.get(name=location_name)
+  ImageStoragePlacement.create(location=location, storage=storage)
+  storage.locations = {location_name}
+  return storage