Don't kill sar prematurally in devstack-gate

This change ensure that 'sar' is not killed by stack.sh when USE_SCREEN=False

Fixes bug #1238482

Change-Id: Id354619a43c27eabbc57f61ba33be2a9493244aa
diff --git a/stack.sh b/stack.sh
index aa0efea..89a03b5 100755
--- a/stack.sh
+++ b/stack.sh
@@ -840,7 +840,7 @@
 # If enabled, systat has to start early to track OpenStack service startup.
 if is_service_enabled sysstat;then
     if [[ -n ${SCREEN_LOGDIR} ]]; then
-        screen_it sysstat "sar -o $SCREEN_LOGDIR/$SYSSTAT_FILE $SYSSTAT_INTERVAL"
+        screen_it sysstat "cd ; sar -o $SCREEN_LOGDIR/$SYSSTAT_FILE $SYSSTAT_INTERVAL"
     else
         screen_it sysstat "sar $SYSSTAT_INTERVAL"
     fi