* Add basic support for hypervisor plugins in lib/nova_plugins
* Add lib/nova_plugins/hypervisor-docker to use Docker as a Nova
hypervisor.
* Add tools/install_docker.sh to install the Docker daemon and
registry container, download base image and import
* Configure Nova to use docker plugin
* Add docker exercise and skip unsupported ones
Nova blueprint: new-hypervisor-docker
Change-Id: I9e7065b562dce2ce853def583ab1165886612227