Converts 'image' to json primitive on compute.rpcapi.prep_resize

The 'image' should be converted to primitive since it contains
complex structures. Today it works because it relies on
scheduler.rpcapi.prep_resize which does the conversion.

This is part of other changes that will move resize/cold migration
to conductor. scheduler.rpcapi.prep_resize will be moved and then
compute.rpcapi.prep_resize has to properly convert parameters.

Partially implements bp cold-migrations-to-conductor

Change-Id: Id89896bfb379ab9eb8bf13c3513b9290d88408fd
Merge "Converts 'image' to json primitive on compute.rpcapi.prep_resize"