replication: refactor the code setting principals as replica bind DNs
    
    In addition to improving the readability of
    `setup_krb_princs_as_replica_binddns` method, the re-usable bits were factored
    out to separate methods
    
    https://fedorahosted.org/freeipa/ticket/6406
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>