Fixes autoscaling with VNF Indicators

Adds vdu_name to VNF Indicators collection.
This fixes autoscaling as Prometheus is queried using alarm
tags, which includes vdu_name.

Change-Id: I68e6368e4839b96737142be426d82d1b1e07e186
Signed-off-by: Gianpietro Lavado <glavado@whitestack.com>