Adds partial support for interface metrics in OpenStack plugin

Adds support for 'packets_in_dropped', 'packets_out_dropped', 'packets_received', 'packets_sent'
All of this metrics correspond to a sum of the metric for each interface in the vdu

Change-Id: I93b4fb760b927f98417ae2e65a27c0c2b3fb5b6f
Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>