Add stestr to available repos
This will allow to test stestr master in Tempest
gate to detect any incompatible change in advance.
Needed-By:
- gnocchixyz/gnocchi
- gtest-org/ansible
- kubernetes-incubator/kubespray
+ - mtreinish/stestr
- novnc/novnc
- openvswitch/ovs
- pytest-dev/pytest-testinfra
