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/.gitignore b/.gitignore
index e625780..7c527a4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -26,7 +26,7 @@
 .coverage
 .tox
 nosetests.xml
-.testrepository
+.stestr/
 .venv
 
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