Add support for Docker as Nova hypervisor

* 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
diff --git a/exercises/boot_from_volume.sh b/exercises/boot_from_volume.sh
index 36524ed..fe27bd0 100755
--- a/exercises/boot_from_volume.sh
+++ b/exercises/boot_from_volume.sh
@@ -44,6 +44,9 @@
 # the exercise is skipped
 is_service_enabled cinder || exit 55
 
+# Also skip if the hypervisor is Docker
+[[ "$VIRT_DRIVER" == "docker" ]] && exit 55
+
 # Instance type to create
 DEFAULT_INSTANCE_TYPE=${DEFAULT_INSTANCE_TYPE:-m1.tiny}