AD: Add a new option to turn off GC lookups
    
    SSSD now defaults to using GC by default. For some environments, for
    instance those that don't or can't replicate the POSIX attributes to
    Global Catalog, this might not be desirable.
    
    This patch introduces a new option ad_enable_gc, that is enabled by
    default. Setting this option to false makes the SSSD contact only the
    LDAP port of AD DCs.