[oslo] Ussuri stable branches created

This has two major implications: 1) Master is no longer feature frozen 2) Any future Ussuri bug fixes will need to be backported Because of 2 and in spite of 1, please try to avoid merging any highly invasive changes to master between now and the Ussuri coordinated release. If we end up needing a last-minute bug fix we don't want to have to disentangle it from a huge refactoring effort (or whatever) in order to backport. Thanks. -Ben