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.
Uses environment inclusions which need tox 2.8.1 due to
a bug fix. So increase the tox minversion.
[1] https://etherpad.openstack.org/p/YVR-python-pti
Change-Id: If6eded0dd802cbdfe49ad72c64004585d4a1484b
