nova-compute will delete all instances if hostname changes

[Dan Smith]

The evacuate code as it is currently in nova will delete instances when instance.host != $(hostname) of the host. This assumes that the instance has been evacuated (because its hostname changed). In that case, deleting the local residue is correct, but if the host's hostname changes, then we will just delete data based on a hunch.

Nova-compute needs a better mechanism to detect if an evacuation has actually been requested before deleting the data.

See Blueprint robustify-evacuate