Update oslo-incubator db.sqlalchemy module

Changes -
 * Use dialect rather than a particular DB API driver
 * Move helper DB functions to db.sqlalchemy.utils
 * Small edits on help strings
 * Transition from migrate to alembic
 * Fix mocking of utcnow() for model datetime cols
 * Add a db check for CHARSET=utf8
 * Remove "vim: tabstop=4 shiftwidth=4 softtabstop=4" from headers

Change-Id: I8782cbb7a2f1a03df6d5c5e8ff6406836a6c645c
Merge "Update oslo-incubator db.sqlalchemy module"