Server Roles: Backend plugin to query roles and attributes
    
    `serverroles` backend consumes the role/attribute instances defined in
    `ipaserver/servroles.py` module to provide low-level API for querying
    role/attribute status in the topology. This plugin shall be used to implement
    higher-level API commands.
    
    https://www.freeipa.org/page/V4/Server_Roles
    https://fedorahosted.org/freeipa/ticket/5181
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Pavel Vomacka <pvomacka@redhat.com>