Bump oslo.db to >= 0.4.0

The latest version of oslo.db sets raise_on_warnings=False for 'MySQL
Connector' database driver that is needed to run migration rules against
that alternative driver.

blueprint enable-mysql-connector

Change-Id: I7dad379967c31dd28d324d2957eca8970801d7df
Merge "Bump oslo.db to >= 0.4.0"