AD: Use ad_site also when site search fails
    
    https://fedorahosted.org/sssd/ticket/2725
    
    Some deployments use the ad_site option for cases where the AD clients
    are not able to find a site for one reason or another. With our current
    code, the ad_site option value can only override a site that the client
    found, not supply the value for cases no site could be found.
    
    This patch fixes the issue.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>