Fix stress runner exit issues

1. The stress runner in parallel run, regardless to the test failures
   returned with 0 (EXIT_SUCCESS) exit status.
2. Stress runner did not stopped run in serial execution, when the stop
   on first failure was enabled.

Change-Id: Ifb71c58a2f676f8a3427160dbc5cfeab61d13b56

tempest/stress/run_stress.py

@@ -87,8 +87,13 @@ def main(ns):
             # NOTE(mkoderer): we just save the last result code
             if (step_result != 0):
                 result = step_result
+                if ns.stop:
+                    return result
     else:
-        driver.stress_openstack(tests, ns.duration, ns.number, ns.stop)
+        result = driver.stress_openstack(tests,
+                                         ns.duration,
+                                         ns.number,
+                                         ns.stop)
     return result