Fix unstack.sh to stop stop_dstat when it is disabled.

unstack.sh always stop dstat process, however unstack.sh show 'dstat: no process
found' when dstat is disabled.
This patch stop function of stop_dstat, when dstat is disabled.

Change-Id: If9054826bed8a7fedd4f77ef4efef2c0ccd7f16e
Closes-Bug: #1435148
diff --git a/unstack.sh b/unstack.sh
index a6aeec5..fdd63fb 100755
--- a/unstack.sh
+++ b/unstack.sh
@@ -173,7 +173,9 @@
     cleanup_trove
 fi
 
-stop_dstat
+if is_service_enabled dstat; then
+    stop_dstat
+fi
 
 # Clean up the remainder of the screen processes
 SCREEN=$(which screen)