Redo virtual_topology call to be faster:
For tbres/resweb (75 nodes): Old time: 42 seconds. New time: 19 seconds. For testbed/BigLan (600 nodes): Old time: 120 seconds. New time: 40 seconds. So, still needs some work, but for the average case its not so bad anymore. I also turn on compression to level 5 by default in our sshxmlrpc.py library, thereby overriding the user's config file for that option.