Make duration in serial test approximate total run time

The duration argument is supposed to mean how long the test will take to
finish. Before this change, if running serially it would take n*duration where
n is the number of test cases specified to run.

Change-Id: I9bd525b17e41fae811e3bf48642b728151e65737

tempest/cmd/run_stress.py

@@ -102,9 +102,11 @@ def main():
                                       call_inherited=ns.call_inherited)
 
     if ns.serial:
+        # Duration is total time
+        duration = ns.duration / len(tests)
         for test in tests:
             step_result = driver.stress_openstack([test],
-                                                  ns.duration,
+                                                  duration,
                                                   ns.number,
                                                   ns.stop)
             # NOTE(mkoderer): we just save the last result code