[US5772] Use a class variable @@all_attributes instead of using fattr.
[US5772] Use a class variable @@all_attributes instead of using fattr.

apptentive-engineering/zuora,zuorasc/zuora,tw5475/zuora,invoice2go/zuora,engineyard/zuora,backupify/zuora,mdemin914/zuora,Sage/zuora,localytics/zuora