relocate instance

Currently, we can use "resize" or "live_migrate" to move an instance to another host. But if the old host down, or the nova-compute service is dead, the action will be failed.
So add a new method named "relocate", to move instance from an host to another, although old host is dead.
The "relocate" only detroy old instance, and create an new one in new host. Not move the disk from old host to new, becasuse if old host is dead, we can't get the disk file from it.