Correctly inherit __table_args__ from parent class

Where unique constraints are added to certain tables by populating
__table_args__, we need to ensure we inherit the top level
__table_args__ that define the default storage engine to use when
creating tables.

Change-Id: Ib886b84922830b60282b4f882591e43f5041e713
Fixes: bug 1288358
Merge "Correctly inherit __table_args__ from parent class"