Update instance.node on evacuate

The evacuate API works like a migration from a dead node. As part of
migration, the instance.node property is updated to reflect the target
node that the instance is migrating to. This is later used when the
migration is confirmed/reverted. The problem is when an instance is
being evacuated, the node property isn't getting updated and the
instance will have the old (dead) node value set on it after it's
evacuated. This patch updates instance.node when the instance is
evacuated.

Fixes bug 1198284

Change-Id: I4901fb937160e955df7c97c37265783e844fdbef
Merge "Update instance.node on evacuate"