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