Correction to incorrect comment
WARNING, FLASE STATEMENT ABOVE. Magnum is most certainly NOT a plugin for Nova. The statement Mr Babcock made applies to the libvirt-lxx virt driver for Nova, which should not be confused with Magnum. Nobody uses libvirt-lxc in produciton, because it does not provide security isolation between neighboring containers that run on the same host. Magnum solves that probelm.
OpenStack Magnum is a new service API for OpenStack that works alongside Nova, and actually uses Heat (OpenStack Orchestration) to create Nova instances that are used to host containers. It's purpose is to give cloud operators a way to offer Containers-as-a-Service will a full multi-tenant solution, a choice of container orchestration software (Kubernetes, Docker Swarm, Apache Mesos/Marathon, etc.), and a choice to run containers on a variety of compute form factors (Virtual Machines, Bare Metal, etc.). Magnum places your containers, and related orchestration software in a cluster called a Bay. It instegrates with Keystone (OpenStack Identity) so you can use the same cloud credentials (username, password, token) you use to interact with Nova to create Magnum Bays for containers, and to create containers (or pods, services, replciation controllers, etc.) in your bays.
Also, see the whitepaper form the OpenStack Foundation: "Exploring Opportunities: Containers and OpenStack."