AD: connect to forest root when downloading the list of subdomains
    
    https://fedorahosted.org/sssd/ticket/2285
    
    Only the forest root has the knowledge about all the domains in the
    forest, the forest leaves only see themselves and the forest root.
    
    This patch switches to connecting to the forest root for downloading the
    trusted domains instead of the server we are connected to.
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    Reviewed-by: Sumit Bose <sbose@redhat.com>