Changes select_destinations to return dicts instead of objects

This patch refactors the select_destinations to return a dict
instead of a HostState object. As the result of this change, no
additional code needs to be added to jsonutils.to_primitive
function.

Partially implements bp cold-migrations-to-conductor

Change-Id: Ic24080766e48abff36bbbe2c04fa5da77621fde5
Merge "Changes select_destinations to return dicts instead of objects"