Don't use ModelBase.save() inside of transaction

We using block 'with session.begin()' to organize some operations in
one transaction. session.begin() returns a transaction instance, then
does some operations, and will commit or rollback automatically
before leaving the block. ModelBase.save() always submit a commit,
and that is not expected.
When we get a persistent object from database, we just modify the
object inside of block 'with session.begin()' and sqlalchemy will
update it,don't need method session.add() or ModelBase.save()

Fixes bug #1224429

Change-Id: I2204b3023c01e65950e6131cd08b4ded5564e9b9
Merge "Don't use ModelBase.save() inside of transaction"