Switch to stestr

According to Openstack summit session [1] stestr is
maintained project to which all Openstack projects
should migrate.
Let's switch it then.

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

Change-Id: I856024f8b66f5edaf40ae7b798fce21c43d021e7
diff --git a/.gitignore b/.gitignore
index 963e589..a678dd2 100644
--- a/.gitignore
+++ b/.gitignore
@@ -27,8 +27,8 @@
 !.coveragerc
 .tox
 nosetests.xml
-.testrepository
 .venv
+.stestr/
 
 # Translations
 *.mo