WebUI: cert login: Configure name of parameter used to pass username
    
    Directive LookupUserByCertificateParamName tells mod_lookup_identity module the
    name of GET parameter that is used to provide username in case certificate is
    mapped to multiple user accounts.
    Without this directive login with certificate that's mapped to multiple users
    doesn't work.
    
    https://pagure.io/freeipa/issue/6860
    
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>