set allocation_pool_id nullable=False

fixes bug 1103216

SQL Primary Keys cannot be null, so this patch fixes an
inconsistency between the models and SQL.  The databases were
correcting this automatically, so a migration is not required.

Change-Id: I724294580de9b86a2edfd4fdc6907e9469b6c552
Merge "set allocation_pool_id nullable=False"