Switch to stestr

According to Openstack summit session [1],
stestr is maintained project to which all Openstack projects should migrate.
Let's switch to stestr as other projects have already moved to it.

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

Change-Id: I8f70f7d8a3d18301559c0eb47e6a64c8b5100d39

tox.ini

@@ -31,8 +31,13 @@ commands = {posargs}
 
 [testenv:cover]
 basepython = python3
+setenv =
+    PYTHON=coverage run --source watcher --parallel-mode
 commands =
-    python setup.py testr --coverage --testr-args='{posargs}'
+    stestr run '{posargs}'
+    coverage combine
+    coverage html -d cover
+    coverage xml -o cover/coverage.xml
     coverage report
 
 [testenv:docs]