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
(cherry picked from commit 3da3dfb733237ed2c908d8770961d8d6656ea1a3)
Merge "Update instance.node on evacuate" into stable/grizzly