Moves compute.rpcapi.prep_resize call to scheduler.manager

This change removes the scheduler_prep_resize method from
the scheduler drivers and put the logic on the scheduler
manager instead.

This is part of other changes that will completely move
cold migration related tasks to conductor. Next change
will move the scheduler manager prep_resize logic to
conductor.

Partially implements bp cold-migrations-to-conductor

Change-Id: Iab6d414acfa32eeb36a2b8cf358ce5948652c49c
Merge "Moves compute.rpcapi.prep_resize call to scheduler.manager"