Return Selection Objects

Before Queens, the scheduler's select_destinations() method returned a dictionary representing the selected host for each requested instance. In Queens, however, this will change to return a list of these dictionaries for each instance, with the first item being the selected host, and the rest being optional hosts in case the initial build fails. What's more, each of these host dictionaries must also include the corresponding allocation dictionary that would be used to claim or unclaim the resources on that host for the instance. We propose to create a new Selection object that would contain these data structures, make accessing their elements simpler and clearer, and document what each field in the object represents.