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: Ifa7bb85e6290898c073859cbaf38f566ece2dff9
Adds some dependencies, tweaks tox.ini to only include envs we care
about, and fixes an issue with the latest diskimage-builder where
it would fail if no operating-system element is included.
Change-Id: I6da9fb4f55978227b81c17a6d552b27ec2b90552