added missing imports in db_tests/__init__.py (for nosetests)
added missing imports in db_tests/__init__.py (for nosetests)


Former-commit-id: 044a43a505df986377fe4eb117fc4efcacd1f0dd [formerly 044a43a505df986377fe4eb117fc4efcacd1f0dd [formerly c8dccac82ce32a72e86de61e35a6b0b384c7ff62]]
Former-commit-id: 96c7aa1e26ebaec8a93a1cedf2a32865ae082b28
Former-commit-id: e0330b1588589e30e46f8e3a57bb399d6a4d9e84
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine