Switch to use stestr for unit test

According to Openstack summit session [1],
stestr is maintained project to which all Openstack projects should migrate.
So we should switch to stestr.

[1] https://etherpad.openstack.org/p/YVR-python-pti

Change-Id: I6c6e1b3873a71d3b069f0c0bc45d9ba9885c2886
diff --git a/tox.ini b/tox.ini
index 13d72c6..3888605 100644
--- a/tox.ini
+++ b/tox.ini
@@ -9,7 +9,7 @@
 setenv =
    VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/test-requirements.txt
-commands = python setup.py test --slowest --testr-args='{posargs}'
+commands = stestr run --slowest {posargs}
 
 [testenv:pep8]
 basepython = python3
@@ -20,8 +20,14 @@
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
-commands = python setup.py test --coverage --testr-args='{posargs}'
+setenv =
+   {[testenv]setenv}
+   PYTHON=coverage run --source tempest_horizon --parallel-mode
+commands =
+   stestr run {posargs}
+   coverage combine
+   coverage html -d cover
+   coverage xml -o cover/coverage.xml
 
 [testenv:docs]
 basepython = python3