Make __table_args__ declarative in RBACColumns

The UniqueConstraint being constructed at class load time for
RBACColumns meant that all tables inheriting from it ended up
sharing the same UniqueConstraint object. This led to a bunch
of warnings about columns being replaced from one table to
another. This didn't appear to affect any functionality but it
may have broken queries across both tables.

This patch just converts it into a declared attr so a separate
constraint object gets created for each table that inherits the
class.

Closes-Bug: #1550618
Change-Id: I02b8e911125c06691bf02b6e7ac02cf25c4c4142

neutron/db/rbac_db_models.py

@@ -16,6 +16,7 @@
 import abc
 
 import sqlalchemy as sa
+from sqlalchemy.ext import declarative
 from sqlalchemy.orm import validates
 
 from neutron._i18n import _
@@ -54,10 +55,12 @@ class RBACColumns(model_base.HasId, model_base.HasTenant):
         # to reference the type. sub-classes should set their own
         pass
 
-    __table_args__ = (
-        sa.UniqueConstraint('target_tenant', 'object_id', 'action'),
-        model_base.BASEV2.__table_args__
-    )
+    @declarative.declared_attr
+    def __table_args__(cls):
+        return (
+            sa.UniqueConstraint('target_tenant', 'object_id', 'action'),
+            model_base.BASEV2.__table_args__
+        )
 
     @validates('action')
     def _validate_action(self, key, action):