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
Merge "Fix problem with never stopping stress tests"