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/__init__.py

@@ -1 +1,14 @@
-from data.model.legacy import *
+class DataModelException(Exception):
+  pass
+
+
+class Config(object):
+  def __init__(self):
+    self.app_config = None
+    self.store = None
+
+
+config = Config()
+
+
+from data.model.legacy import *