Replace sh sleep with native gfsh sleep command

Author: JinwooHwang

wan/scripts/start.gfsh

@@ -22,7 +22,7 @@ disconnect
 # Start the New York (ny) cluster
 run --file=./scripts/start-ny.gfsh
 disconnect
-sh sleep 5
+sleep --time=5
 
 # Create the region on each cluster and bind the gateway senders/receivers to region
 run --file=./scripts/start-wan.gfsh