Samba Enterprise From Sernet No Longer Free
http://www.samba.plus/
Currently this, other than rolling your own, this appears the only way to have a Samba AD on CentOS.
Later in the Samba 4.3 effort, MIT Kerberos is suppose to be supported so it MAY be easier to roll your own in a while, but until then, things are as they are.
I am rethinking my efforts to develop a ARM based AD. I may just fall back (as everyone else) to just an NT style PDC.
One thought on - Samba Enterprise From Sernet No Longer Free
For the time being you can still get samba 4.2 packages and run Samba AD on CentOS @ https://portal.enterprisesamba.com.
The paid service is only for 4.3+ so you should be good for a while.
HTH,
Regards,