Converts scheduler.utils.build_request_spec return to json primitive

Other than 'instance', 'image' can also be a complex structure and
then it needs to be converted. See: nova.compute.rpcapi.prep_resize
as an example.

Change-Id: I54570dad3eae6ecc91a433af68263794c22e268d
Merge "Converts scheduler.utils.build_request_spec return to json primitive"