os-testr has moved over to use stestr instead of testr. While this
is usually compatible with existing settings, there is a warning
that is emitted when .stestr.conf is not present. It is usually
able to fall back to parsing the .testr.conf file, but to be more
correct and to prevent future problems we should update the config.
Change-Id: I10446c539aaf9edd8b52b17776e08c12757def31
Depends-On: I98a296d009e64d6afab02a53bd1af878f78c4321