Add retry decorator to backend-frontend requests.
    
    This is an addendum to 251c008372a3.
    
    There, a retry decorator was added to try and work around some DNS issues when
    we can't even establish a connection.  We covered connections from
    greenwave/web to resultsdb and waiverdb, but I forgot about greenwave's
    connection from the backend to greenwave/web.  This covers that third case.
    
    It gets tricky like the comments says because the backend doesn't have access
    to the frontend configuration - no application context.  We should think about
    some kind of unified config for a future change.