Sync oslo db.sqlalchemy.utils to nova

Changes from oslo-incubator:
===============================

Change I9c441d26: Use single meta when change column type
Change Id6cf7b12: Helper function to sanitize db url credentials
Change I34000ad2: Add function drop_unique_constraint()
Change I292ec445: Change sqlalchemy/utils.py mode back to 644
Change Ia0366dbd: Raise ValueError if sort_dir is unknown
Change Ie54fcd5d: Migrate sqlalchemy utils from Nova

Change-Id: Ia84de322cf9dcf30624ce64f373274ca2e13ae14
Merge "Sync oslo db.sqlalchemy.utils to nova"