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.
This patch also moves "commands" and "setenv"
from [testenv:pyNN] sections to [testenv].
[1] https://etherpad.openstack.org/p/YVR-python-pti
Change-Id: Ic9c99015516c31cbda2a626f8df4ff2f8dc3d852
Vu Cong Tuan