Handle ibm_db_sa DBDuplicateEntry integrity errors

This patch adds duplicate entry integrity error handling for the
ibm_db_sa driver to the common sqlalchemy db session.

Closes-Bug: #1227321
Part of nova blueprint: db2-database

Change-Id: I2f0d3d660373e4f647615f121fed0da9442f8cd2
Merge "Handle ibm_db_sa DBDuplicateEntry integrity errors"