certmonger: avoid mutable default argument
    
    certmonger._get_requests has a mutable default argument.  Although
    at the present time it is never modified, this is an antipattern to
    be avoided.
    
    In fact, we don't even need the default argument, because it is
    always called with a dict() argument.  So just remove it.
    
    Part of: https://pagure.io/freeipa/issue/8186
    
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>