changed user principal from id to username

This decouples a order model more from a user

ordr2/models/user.py

@@ -47,7 +47,7 @@ class User(Base):
     @property
     def principal(self):
         ''' returns the principal identifier for the user '''
-        return 'user:' + str(self.id)
+        return 'user:' + self.user_name
 
     @property
     def role_principals(self):