Template / VM deletion script updates

This pulls in a script that has been running on the template uploader server, and adds some functionality for deleting older templates and VMs. Currently, it is set to retain the 3 most recently uploaded nightly templates and delete anything older than that. The constant for the number of builds to retain is defined by MRU_NIGHTLIES. Some of the pre-existing code should be DRYed up some more, but it functions correctly and will run as an hourly cron job on the template uploader box.