Convert last use of direct database instance fetching from network api

This is the last db.instance_*() call in network/api.py. This patch
converts it to use the Instance object.

Related to blueprint compute-manager-objects-juno

Change-Id: Id7897da6daa3d76a95403466be17f9eaef8b7850
Merge "Convert last use of direct database instance fetching from network api"