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: Iee13deb54056e1c81d3732cb51a61c68481607fa

tox.ini

@@ -10,7 +10,7 @@ setenv =
    VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-commands = python setup.py testr --slowest --testr-args='{posargs}'
+commands = stestr run --slowest {posargs}
 
 [testenv:pep8]
 commands = flake8
@@ -19,7 +19,14 @@ commands = flake8
 commands = {posargs}
 
 [testenv:cover]
-commands = python setup.py testr --coverage --testr-args='{posargs}'
+setenv =
+    {[testenv]setenv}
+    PYTHON=coverage run --source cloudkittydashboard --parallel-mode
+commands =
+    stestr run {posargs}
+    coverage combine
+    coverage html -d cover
+    coverage xml -o cover/coverage.xml
 
 [testenv:docs]
 commands = python setup.py build_sphinx