Upddate tox config

- Removes py3 env since we don't run unit tests
- Update requirements to use stestr and coverage
- Update tox.ini to use stestr

Change-Id: Iecb18a3e616173217b7edb756dcaabd8df009629
diff --git a/test-requirements.txt b/test-requirements.txt
index 9292937..60a3752 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -4,6 +4,9 @@
 
 hacking!=0.13.0,<0.14,>=0.12.0 # Apache-2.0
 
+stestr>=1.0.0 # Apache-2.0
+coverage!=4.4,>=4.0 # Apache-2.0
+
 sphinx!=1.6.6,!=1.6.7,>=1.6.2 # BSD
 openstackdocstheme>=1.20.0 # Apache-2.0