Make offline LDIF modify more robust
    
    * move code to installutils
    * add replace_value method
    * use lists instead of single values for add_value, remove_value methods
    
    https://fedorahosted.org/freeipa/ticket/4949
    
    Also fixes:
    https://fedorahosted.org/freeipa/ticket/4048
    https://fedorahosted.org/freeipa/ticket/1930
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>