Don't use a specific arg list for stress in tox.ini

Callers should be able to specify the arguments.

Change-Id: I1d60af5e5300ff91ce3777ac46df655442098013
diff --git a/tox.ini b/tox.ini
index 7abe989..262a27e 100644
--- a/tox.ini
+++ b/tox.ini
@@ -86,7 +86,7 @@
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 commands =
-    run-tempest-stress -a -d 3600 -S
+    run-tempest-stress '{posargs}'
 
 [testenv:venv]
 commands = {posargs}