as described in
https://governance.openstack.org/tc/reference/pti/python.html
projects should use 'stestr' as unit tests runner.
This patch moves unit and coverage tests to use stestr as test runner
instead of nose.
Change-Id: I8508caa6ea08c5c98b67f34be7f61417df2d7c1e
Pavlo Shchelokovskyy