Garbage collection image+storage callback support

Add support to GC to invoke a callback with the image+storages removed. Only images whose storage was also removed will be sent to the callback. This will be used by security scanning for its own GC in the followup change.

data/model/__init__.py

@@ -107,6 +107,10 @@ class Config(object):
   def __init__(self):
     self.app_config = None
     self.store = None
+    self.image_cleanup_callbacks = []
+
+  def register_image_cleanup_callback(self, callback):
+    self.image_cleanup_callbacks.append(callback)
 
 
 config = Config()