When will Bitdefender be an option in the Agent install?

Should be included with 2018.3 going forward, from what I've seen.

Granted, "subject to change" and all that, but somewhat unlikely that'll get yanked unless there's some earth-shatteringly good reason for it.

Current plan is to have it in 2018.3 which is due out this month if everything goes as planned. It will also be included in SU6 for 2017.3 which will be be worked on after 2018.3 is released.