dsinstance, httpinstance: consolidate certificate request code
    
    A different code path is used for DS and httpd certificate requests in
    replica promotion. This is rather unnecessary and makes the certificate
    request code not easy to follow.
    
    Consolidate the non-promotion and promotion code paths into one.
    
    https://pagure.io/freeipa/issue/6757
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>