backend: work with multiple CDN hostnames per instance
    
    It seems that each instance has automatically generated CDN hostname
    which looks like this
    
        d1nld9ovj32u75.cloudfront.net
    
    But at the same time it can have an assigned hostname, e.g.
    
        download.copr.fedorainfracloud.org
    
    And in the S3 access logs, there are accessess for both
    hostnames (even mixed within one access log)
    
    Merges: #2329