If a scheduler RPC message isn't handled directly by a SchedulerManager
method, the __getattr__() fallback passes the message to a driver
method in the form of schedule_${method}() and, if that doesn't exist,
instead calls the schedule() method supplying the topic and method
args.
This is pretty bizarre stuff and we appear to only use it in two cases:
1) live_migration - this is how the schedule_live_migration()
method in the driver gets called, but the side-effect is that
we require the client to pass a topic argument which is never
used. This would be much more sanely handled with an explicit
SchedulerManager.live_migration() method.
2) create_volume - the volume API asks the scheduler to pick a
target host for create_volume() using this method. This would
be easily handled with an SchedulerManager.create_volume()
method.
Change-Id: I1047489d85ac51d8d36fea1c4eb858df638ce349
ee344a3b8c