Migrate to stestr

This patch updates tox.ini to use stestr instead of testr.

Change-Id: I6be57c0c90d7711c172243b9de43034735334de7
diff --git a/test-requirements.txt b/test-requirements.txt
index 8ad5962..47c128f 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -7,6 +7,6 @@
 coverage!=4.4,>=4.0 # Apache-2.0
 python-subunit>=1.0.0 # Apache-2.0/BSD
 oslotest>=3.2.0 # Apache-2.0
-testrepository>=0.0.18 # Apache-2.0/BSD
+stestr>=2.0.0 # Apache-2.0
 testscenarios>=0.4 # Apache-2.0/BSD
 testtools>=2.2.0 # MIT