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
Revert "Initial scheduler support for instance_groups"

This reverts commit b2c17ae891b12118b9d11bf0ecc44e77956715c8.

While reviewing merged changes, I came across this one.  It's using the
conductor in a way that it shouldn't.  The scheduler has db access, so
there's no need to add the extra conductor overhead.  The best thing to
do here is to just revert for now, IMO.

Change-Id: Iaa0d67df7bb84913784d925cffebcd5f252249ea
Merge "Revert "Initial scheduler support for instance_groups""
Merge "Remove db session hack from conductor's vol_usage_update()"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp