This patch addresses the issue by removing the extra nesting
(which is effectively redundant).
The longer story about this patch is the following:
Race conditions may cause DBDuplicateEntry exceptions that
require a transaction to be rollbacked back, and yet make
the whole operation succeed. A classic example is what has
been solved in commit fbc6b99. If the rollback is done in a
nested transaction, the above mentioned exception is raised.
To address the problem, we could use savepoints by means of
sqlalchemy's begin_nested(); Even though this approach is
preferable, it causes quite a bit of changes in the unit
tests (because of sqlite); it may also require that certain
DBMS, or certain DB backend configurations, support savepoints.
In the end, the simpler approach was chosen.
Closes-bug: #1354072
Change-Id: Ic9393319e55f71d681124bd3052e939724ebab6b
cf69c5b4c3