Use only latest repo from Pulp
    
    If Pulp returns multiple repos for a given content set, and all of them
    have a single product version each, and the product version follows the
    same schema for all repos, we can use only the latest one.
    
    This is based on the assumption that X.Y+1 contains all the content from
    X.Y, so there's no need to merge all the repodata.
    
    This should hopefully make pulp composes faster and avoid issues where
    mergerepo_c is running out of memory.
    
    Fixes: https://pagure.io/odcs/issue/357
    JIRA: COMPOSE-4231