create-cluster will try to create two clusters with the same name

[fnothaft]

I observed this tonight, with the two clusters separated by a few hours. I got distracted and thought I had terminated the first cluster. cgcloud started creating the master; I stopped it before it got to the worker. I'm not sure how hard it would be to check for the existence of another cluster with the same name before starting a second cluster; it seems like something that would be useful to have.