Bug sqlalchemy load_db_driver hook for web.py

Problem: Sqlalchemy session fails with InvalidRequestError
in case of errors during flush

Solution: I found that only reason why it can fail with
InvalidRequestError is need for rollback and in case of web.HTTPError it doesnot happend
So rollback session without any special cases (e.g. web.HTTPError)

Change-Id: Ic530528278df1245cda4bbd27309232cf7e567a6
Merge "Bug sqlalchemy load_db_driver hook for web.py"