I need a way to manage LXC containers on Ubuntu Server 12.04 - similar to what Proxmox is. Googling indicates that openstack is one option - would this work?
OpenStack can manage virtual machine instances, and LXC deployments on bare metal.
It might be overkill for your scenario though, it requires a database server, message qeueue server, and many components to be configured (nova, swift, keystone, horizon, etc.).
If you do decide to try OpenStack, I suggest you get started with the devstack scripts:
Simple webpanel for Ubuntu from 12.04 is http://lxc-webpanel.github.io/.