Only allow admins to delete VMs from down compute nodes

Doing a local delete of a VM (removing it from nova even though the compute node is down) is a very handy operation, but we've run into some issues with allowing just anyone to do it. The main problem that we've seen first hand is when users deleted their VMs from a down compute node (which put those VMs' IPs back into the available pool) and then new VMs are assigned those same IPs. If the compute node comes back up and the old VMs restart, we now have old and new VMs both thinking they have that particular IP. Not a Good Thing. We've done a local patch so that only admins are allowed to do local deletes. This keeps the useful functionality, but puts it in the hands of the folks who know if the compute node is ever going to come back or not. We've found this extremely useful and figure that others may find it helpful as well.