Add a uniqueness hash to derived image storage to break caching over tags

This allows converted ACIs and squashed images to be unique based on the specified tag.

Fixes #92

data/database.py

@@ -617,12 +617,13 @@ class DerivedStorageForImage(BaseModel):
   source_image = ForeignKeyField(Image)
   derivative = ForeignKeyField(ImageStorage)
   transformation = ForeignKeyField(ImageStorageTransformation)
+  uniqueness_hash = CharField(null=True)
 
   class Meta:
     database = db
     read_slaves = (read_slave,)
     indexes = (
-      (('source_image', 'transformation'), True),
+      (('source_image', 'transformation', 'uniqueness_hash'), True),
     )