Make _get_instance_nw_info() use Instance object

This makes the _get_instance_nw_info() method use the Instance
object instead of a direct conductor query to do its job.

Related to blueprint compute-manager-objects

Change-Id: I53cc2e7844d5c4a43c6bb4c08c97f1547e5e6984
Merge "Make _get_instance_nw_info() use Instance object"