Fix problem with never stopping stress tests

This fix will fix the issue for never stopping stress test in
the nightly build jobs. It changes three problems related to this:

 - An exception in tearDown() will not cause a endless loop
 - Hanging processes will be killed with -9 after a while
 - tox job will stop on first error

Fixes-bug: 1230357
Change-Id: I3a0160295d98fdc7f8ffce64cc4a08c5c8ae654a
diff --git a/tox.ini b/tox.ini
index 1b8a0fd..abc9e42 100644
--- a/tox.ini
+++ b/tox.ini
@@ -86,7 +86,7 @@
 sitepackages = True
 setenv = VIRTUAL_ENV={envdir}
 commands =
-    python -m tempest/stress/run_stress -a -d 3600
+    python -m tempest/stress/run_stress -a -d 3600 -S
 
 [testenv:venv]
 commands = {posargs}