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: I15fd3427485ff26ee936cae622585745ba9281b7
Sean McGinnis