os-testr has moved over to use stestr instead of testr. While this
is usually compaible 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: I929f75d069f20b8cfa8af4e0712883e91a54dc4f
Rui Yuan Dou