Scheduler API clean up and refactor

Scheduler API clean up and refactor

Unlike Nova scheduler, which has to consider serving compute and volume
scheduling, Cinder scheduler only serves volume scheduling, so there's no
need to keep generic interface 'schedule'. Instead, 'schedule_create_volume'
is added (if missing) to manager/driver class and chance/simple scheduler
driver implementation.

Also this patch changes the interface between API service and scheduler to
allow more information about volume is passed to scheduler for advanced
scheduling while maintained backward compatibility. And this change bumps
scheduler RPC API to version 1.2.

Change-Id: I42be05675cd73f89a03c84105ec512d7ee4f3c3a