According to Openstack summit session [1],
stestr is maintained project to which all Openstack projects should migrate.
Let's switch to stestr as other projects have already moved to it.
[1] https://etherpad.openstack.org/p/YVR-python-pti
Change-Id: I0ec7bc7926e57b243e38b2c696815e648d0b3f61
Vu Cong Tuan