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: Ica0262653e8c18681d049acca6d5920ac38f97e6
Vu Cong Tuan