Don't need session.flush in context managed by session

Within the scope of a single method, keeping all the reads and writes
within
the context managed by a single session. In this way, the session's
__exit__
handler will take care of calling flush() and commit() for you.
If using this approach, you should not explicitly call flush() or
commit().
See http://docs.sqlalchemy.org/en/rel_0_7/orm/session.html#committing

Change-Id: I8e49179cb6cb71fc7b7c06799adcf9f10164a09b
Partial-Bug: #1258044
Merge "Don't need session.flush in context managed by session"