Add support for Docker as Nova hypervisor

This is in support of Nova blueprint new-hypervisor-docker
https://blueprints.launchpad.net/nova/+spec/new-hypervisor-docker

* Add basic support for hypervisor plugins in lib/nova_plugins
* Add lib/nova_plugins/hypervisor-docker to use Docker as a Nova
  hypervisor.
* Install Docker PPA and install Docker
* Configure Nova to use docker plugin
* Download base image and import
* Add docker exercise and skip unsupported ones

Change-Id: I9e7065b562dce2ce853def583ab1165886612227
Merge branch 'docker' into dt-test