Check instance exists or not when evacuate

When evacuate instance to another host, similar to instance create,
nova compute must make sure the instance not yet exist on the host.

Currently there is no check for this, the fix was add the checking
of instance exist or not when evacuate.

Fix bug 1216501

Change-Id: I3ccbd5db5174904e17ffa8f9236ca4e1a09f4dbe
Merge "Check instance exists or not when evacuate"