cleanup to objects.fields

Since 'AutoTypedField' has been redefined in scope of objects.fields,
we should use local definitions.
This is better for code consistence by using same coding style.

Change-Id: I4d37d6b093e56f85a5a5de8d81b6bb0411a8f73c

nova/objects/fields.py

@@ -1274,7 +1274,7 @@ class InstancePowerStateField(BaseEnumField):
     AUTO_TYPE = InstancePowerState()
 
 
-class ListOfListsOfStringsField(fields.AutoTypedField):
+class ListOfListsOfStringsField(AutoTypedField):
     AUTO_TYPE = List(List(fields.String()))
 
 
@@ -1284,5 +1284,5 @@ class ListOfUUIDField(AutoTypedField):
     AUTO_TYPE = List(fields.UUID())
 
 
-class DictOfSetOfIntegersField(fields.AutoTypedField):
+class DictOfSetOfIntegersField(AutoTypedField):
     AUTO_TYPE = Dict(Set(fields.Integer()))