Add separate attribute to store trusted domain SID
    
    We need two attributes in the ipaNTTrustedDomain objectclass to store different
    kind of SID. Currently ipaNTSecurityIdentifier is used to store the Domain-SID
    of the trusted domain. A second attribute is needed to store the SID for the
    trusted domain user. Since it cannot be derived safely from other values and
    since it does not make sense to create a separate object for the user a new
    attribute is needed.
    
    https://fedorahosted.org/freeipa/ticket/2191