cert: add object plugin
    
    Implement cert as an object with methods rather than a bunch of loosely
    related commands.
    
    https://fedorahosted.org/freeipa/ticket/5381
    
    Reviewed-By: David Kupka <dkupka@redhat.com>
    Reviewed-By: Pavel Vomacka <pvomacka@redhat.com>