Make rollback logic more robust
Make rollback logic more robust

SQLAlchemy does complex housekeeping tasks when session.rollback() is
called. If that fails for whatever reason, we should not use the session
again because that could flush out bogus changes.

Change-Id: I5aeb4c0d262b58f3269a273e968a32675dc42c85

Reviewed-By: 790b96b72b5ebbbfb73a2caa0f14d8894a9e0fd7@morganstanley.com

quattor/aquilon,stdweird/aquilon,stdweird/aquilon,stdweird/aquilon,guillaume-philippon/aquilon,quattor/aquilon,guillaume-philippon/aquilon,guillaume-philippon/aquilon,quattor/aquilon