AD: Do not remove non-root domains when looking up root domain
    
    https://fedorahosted.org/sssd/ticket/2322
    
    When the AD subdomains code looked up the root domain subsequently
    (after the domain list was already populated), the non-root domains
    might have been removed along with their respective tasks, because the
    root domain lookup only ever matched a single root domain.
    
    This could cause havoc especially during login when different lookups
    for different domains might be going on during user group refresh.
    
    Reviewed-by: Pavel Reichl <preichl@redhat.com>