This commit is a follow-up of "switch to using stestr"
which was merged already .
After switch to using stestr, testrepository is unnecessary
and should be removed.
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.