Fix tox stress invocation to not quote arguments

Change-Id: I6fc1e64c3e52a02126235d5cc42831ee2d25f997
diff --git a/tox.ini b/tox.ini
index 9f52f0d..f75e868 100644
--- a/tox.ini
+++ b/tox.ini
@@ -91,7 +91,7 @@
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 commands =
-    run-tempest-stress '{posargs}'
+    run-tempest-stress {posargs}
 
 [testenv:venv]
 commands = {posargs}