Improve handling of server deletions
Improve handling of server deletions. * Have the API return a server's state as DELETED when the deleted flag has been set in the VirtualMachine model. This makes the server immediately invisible, instead of waiting for an indefinite amount of time for the Ganeti backend to remove the VM. * Set the deleted flag in the VirtualMachine model when Ganeti reports successful removal of an instance. This catches instance deletions initiated by the admin at the Ganeti backend.