[qa] Retire os-testr

Hey, I think it's time we retired os-testr. It is already effectively unmaintained and we made a concerted effort some cycles ago to migrate all projects to stestr. I think it's time we finally closed that out. For any projects still using os-testr, stestr effectively provides a drop-in replacement. I'll propose patches to do this shortly. Please let me know if anyone has any concerns about this course of action. Cheers, Stephen