If you're open to CentOS, you can cluster with ricci/lucci that comes with CentOS.
(This comment got deleted for some reason for not answering the original question so I will elaborate)
You are asking about clustering 100 machines, they currently run ubuntu linux. And to EASILY make a big cluster
For a BIG cluster, my response is if you are able to move to CentOS (free) it comes with ricci/luci which is clustering software and you should be able to cluster 100 machines. I'm not sure if ubuntu clustering can handle that. So my first thing would be to ask them if I can migrate over to CentOS.
yum install ricci
yum install luci
Start the services and you can add nodes to your cluster just like that very EASILY. Go to http://[node]:8084 of any server with ricci installed and running.
From here you can add all your nodes in - the web gui is pretty easy to manage. The hard part is adding a cluster disk
I didn't answer your specific question about Ubuntu except to say I think if you want easy and your technology is open to a migration that CentOS is free and pretty easy in that respect.
Hope that explains more my answer.