host/service-show/find shouldn't fail on invalid certificate
    
    host/service-show/find methods would have failed if the first
    certificate they had in userCertificate attribute were invalid.
    Expected behavior is that they just show the rest of the reqested
    attributes.
    
    https://fedorahosted.org/freeipa/ticket/5797
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>