Add model_query() to db.sqlalchemy.utils module

Added query helper - model_query() function.
There are similar functions in Nova, Cinder, and other projects, so it
makes sense to move this helper to common db code.

POC in Nova - Ic14d2214c8ce16d6b22c6f31511aa6c0838aa17a

Change-Id: I86e78eadc5a76d47452dfefbd4faef697d613b50