ssh: add option ssh_use_certificate_keys and enhance man page
    
    This patch adds on option to switch the extraction of ssh keys from
    X.509 certificates on and off and improves the general documentation
    about this feature in the sss_ssh_authorizedkeys man page.
    
    Related to https://pagure.io/SSSD/sssd/issue/3688
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>