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/.stestr.conf b/.stestr.conf
new file mode 100644
index 0000000..473b6e2
--- /dev/null
+++ b/.stestr.conf
@@ -0,0 +1,3 @@
+[DEFAULT]
+test_path=${OS_TEST_PATH:-./ironic_tempest_plugin}
+top_dir=./