The ostestr tox job was recently removed from the tempest tox.ini,
mostly because we're moving towards standardizing on just using stestr
for everything and dropping the dependency on os-testr. During the
review that removed the job, some concerns were raised that it might
break people. To address that this commit adds a release note explaining
the potential upgrade implications and how to work around the removal.
Change-Id: Ibc59e01af9f5a9c4d8c67fbde1f029decf602226