Moves compute.rpcapi.prep_resize call to conductor

This patch moves the prep_resize logic out of scheduler.manager
and creates it on conductor.manager.ComputeTaskManager.

Partially implements bp cold-migrations-to-conductor

Change-Id: I8458071fb13505f55f8d2155c9ec5ee938b1005f
Merge "Moves compute.rpcapi.prep_resize call to conductor"