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: I3fb32622b029bfadef7f34e973d8277c107112f0
diff --git a/.stestr.conf b/.stestr.conf
new file mode 100644
index 0000000..ecc147c
--- /dev/null
+++ b/.stestr.conf
@@ -0,0 +1,3 @@
+[DEFAULT]
+test_path=./barbican_tempest_plugin/tests
+top_dir=./