Node replace iterates through all inputs treating key/value pairs as
UUIDs to nodes. However, when the cluster has policy it injects a key
called last_op into self.inputs in Action.policy_check, treating this as
a pair of UUIDs causes node replace to fail. Therefore we move the nodes
into inputs['candidates'] and only iterate through those.
Change-Id: I14c6c591e3f6cec2c0600a5d9726c7720256d647