Clean up autoinstaller's cluster_*.iso files automatically

[celskeggs]

Currently, the autoinstaller creates a new installer image for each new install, and never cleans it up, so you quickly find yourself with a collection of junk .iso files:

$ ls
[...]
cluster-17081.iso
cluster-24722.iso
cluster-24798.iso
cluster-24946.iso
cluster-25023.iso
[...]

We should automatically delete these files after we're done using them, to keep the working directory clean.