Add support for the Nova Docker driver

This changeset enables support for using Nova's Docker driver. This
change is intended to use the default driver as found in the Nova
source code. This will likely get augmented to also make use of the
nova-docker driver as found on Stackforge.

These changes are do a few things:
- install the docker-registry as a controller service
- adds an ad-hoc playbook to upgrade precise for Docker
- installs docker
- adds nova user to the docker group
- updates glance to support 'docker' container format

This does not yet handle HA-ness, but that is coming. Some of the
haproxy code needs to be refactored to support this better.