ssh: skip invalid certificates
    
    Current an invalid certificate cause the whole ssh key lookup request to
    abort. Since it is possible that e.g. the LDAP user entry contains
    certificates where the client does not have the needed CA certificates
    for validation we should just ignore invalid certificates.
    
    Resolves https://fedorahosted.org/sssd/ticket/2977
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>