Return to using network_api directly for migrations

Previously, we added indirection APIs in conductor to wrap network
methods that still had direct-to-database calls that compute
manager couldn't make. Now that those are cleaned up, we can
remove the use of the indirection layer (and the layer itself from
conductor) and go direct to network_api again.

Related to blueprint compute-manager-objects-juno

Change-Id: Ia76552f49adf9b0d0128f08e3f99953560d8aa64
Merge "Return to using network_api directly for migrations"