Option to hand out IP addresses from a pool

[geofft]

IP addresses and disk space are the two main resources that are wasted on machines not powered on, and we have plenty of disk space at the moment. There should be an option to let your machine have a dynamic IP address out of a pool of a couple dozen, if you don't expect to use it in a way that needs a static IP. If it's running, we can resolve SERVER.xvm.mit.edu to its current IP, a la dyn.mit.edu.