stestr [1] is a fork of the testrunner python test runner.
Coverage tests can be run using 'tox -e cover'.
Adds a job for checking coverage to Zuul check and gate.
[1] http://stestr.readthedocs.io/en/latest/
Change-Id: I25cd407677a4013d022f87d124c6db43ab6bb2e0
Story: #2001637
Task: #6648
Mark Goddard