Moves scheduler.rpcapi.prep_resize call on compute.api to conductor

This change moves the scheduler.rpcapi.prep_resize from
compute.api.resize to conductor and it calls
conductor.api.migrate_server instead.

This is part of other changes that will completely move cold
migration to conductor.

Next change will move compute.rpcapi.prep_resize and all
related logic from the scheduler to conductor.

Partially implements bp cold-migrations-to-conductor

Change-Id: I2eaff7e94c77deb03a6843f316e243b284e31666
Merge "Moves scheduler.rpcapi.prep_resize call on compute.api to conductor"