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/.stestr.conf b/.stestr.conf
new file mode 100644
index 0000000..057bf50
--- /dev/null
+++ b/.stestr.conf
@@ -0,0 +1,3 @@
+[DEFAULT]
+test_path=./tempest_horizon/tests
+top_dir=.