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

Closes-Bug: #1258044
Change-Id: I1125c72712203f1594ef245761dbfc25ed31eab2
Merge "Don't need session.flush in context managed by session"