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: Iddce63084a10cbc5b1670f42acae6c2ee96e0ce8
Vu Cong Tuan