Refactor Test Base Class for oslotest

Deprecate the test base class and move its features into fixtures.

Provide application-agnostic logging parameters in format strings for oslo.log

The default logging_context_format_string and logging_default_format_string include "instance" which is not useful in all projects. We should do something like what we did with "user_identity" and provide a generic name, which the projects can fill in with their desired value. Discussed at the Juno summit: https://...