Clean/leave OpenStack after a stress test

New parameter "leave_dirty_stack" available in tempest.conf
to prevent (or not) the execution of tearDownClass()
after an exception during a stress test

Closes-Bug: #1273245
Change-Id: I89ecac1fb428cf8809a3634869f2dfdf564aea64
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 48ca905..5ff6d23 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -748,6 +748,11 @@
 # value)
 #default_thread_number_per_action=4
 
+# Prevent the cleaning (tearDownClass()) between each stress
+# test run if an exception occurs during this run. (boolean
+# value)
+#leave_dirty_stack=false
+
 
 [telemetry]