Pull dstat logic into its own function so grenade can use it

In order to use dstat on the new side of grenade, pull dstat code into
its own lib with the function start_dstat

Change-Id: I5c908d594a6f3a90ed4b3f744002bf606841cf07
diff --git a/unstack.sh b/unstack.sh
index fe5fc77..0457ef2 100755
--- a/unstack.sh
+++ b/unstack.sh
@@ -162,6 +162,8 @@
     cleanup_trove
 fi
 
+stop_dstat
+
 # Clean up the remainder of the screen processes
 SCREEN=$(which screen)
 if [[ -n "$SCREEN" ]]; then