stestr is recommended by OpenStack Project Testing Interface for running tests
[1]. This change migrates to stestr for running unit tests and coverage
job. Modified `cover` env allows running it as Zuul job and visualising
its results in Gerrit.
[1] https://governance.openstack.org/tc/reference/pti/python.html#python-test-running
Change-Id: I9e8b79d7a18cd36946daff10abb1a2f43b1aa3a1