Run database tests within long-lived schemas and transactionalized containers

This feature will overhaul the system by which oslo.db provides connectivity and fixture support for database-enabled tests. The new features will allow tests to be run against any number of database backends, to allow "opportunistic" backend URLs to be specified on a per-environment basis, as well as to allow any number of successive tests to run against a fixed schema that is maintained across tests without the need to rebuild the schema on each test.

The key aspects of the feature include that tests run within transactions, that a system is devised which can maintain the scope of a set of schema objects as well as anonymously-generated databases across tests, and that the opportunistic test feature can make use of test scenarios.