[SSSD] AD: read flat name and SID of the AD domain

[Sumit Bose]

Hi,

this patch reads the flat name and the SID from AD and stores then to
the cache. It fixes https://fedorahosted.org/sssd/ticket/1468 directly
and the flat name is needed e.g. for
https://fedorahosted.org/sssd/ticket/1648 and the SID e.g. for
https://fedorahosted.org/sssd/ticket/1558 .

bye,
Sumit