I have had a Cluster since being on 8.x. I have about 900 APs across 2 7220 controllers and a VM Mobility Master controlling the two hardware controllers.
For the DNS discovery method, you just need to create a DNS record called aruba-master in the DNS space that your APs live in. So, if your APs create DNS records in testcorp.loc when they boot up, then put your DNS record there.
I understand that this may be considered an "older" method by some people's opinions... but I never have an issue with it. Not sure if some of the other methods are being blocked by an ACL or something?
On some older versions of 8.x, the Cluster was a little clunky... but on 8.3.0.4 and 8.3.0.5, it seems great so far.
iann miller