> I don't think there is another notable, proven, open source clustered manager of both actual VMs and containers.

Googles Ganeti is strikingly similar to Proxmox, to the point where I suspect Proxmox started as a reimplementation:

http://www.ganeti.org/ (developed at https://github.com/ganeti/ganeti)

It does not come with a GUI, though there are a few third party ones. It has API endpoints for everything, a fully featured CLI client, and is extremely stable.

Googles marketing department does not want you to know this, but Ganeti runs most of its internal infrastructure (not public facing stuff).

No affiliation, just a happy user.