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

.stestr.conf

@@ -1,4 +1,4 @@
 [DEFAULT]
-test_path=${OS_TEST_PATH:-./watcher/tests}
+test_path=./watcher/tests
 top_dir=./