adtrust-install: Mention AD GC port 3286 in list of required ports.
    
    Port name "msft-gc" is taken form /etc/services file provided by package
    setup-2.10.1-1.fc24.noarch.
    
    https://fedorahosted.org/freeipa/ticket/6235
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>