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 change also fixes a bunch of D001 violations where lines are too
long in .rst documents.
[1] https://etherpad.openstack.org/p/YVR-python-pti
Change-Id: Ic20ae60d9020896690c5e7f07124d7500ffd3d2d