Remove db session hack from conductor's vol_usage_update()

Commit 3cf4cb2534c387f287f118d4185d54b1435d3bc0 made the
vol_usage_update() in conductor hack around a stale sqlalchemy
object issue by keeping session active in the current context
until the notification was completed. This layering violation
led to some "creative" tests to try to verify this behavior.

This removes that and refreshes the object before letting it
out of the db/api layer so that all the attributes are present,
thus avoiding the dynamic load during notification.

Change-Id: Ia6ef5cb7f462ff72d170f3af33f2d020ee4d3e3c
Merge "Remove db session hack from conductor's vol_usage_update()"