Change select_destinations() to use request spec

Specification to change the `select_destinations()` scheduler RPC API
method to use a `nova.objects.request_spec.RequestSpec` object instead
of a nested dict.

Co-Authored-By: Sylvain Bauza <sbauza@redhat.com>

Change-Id: I9f9a5d48b1203c619b2e9033a9b65934811c996f
Blueprint: sched-select-destinations-use-request-spec-object