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/unstack.sh b/unstack.sh
index 2268b90..f053bcd 100755
--- a/unstack.sh
+++ b/unstack.sh
@@ -65,6 +65,14 @@
     fi
 fi
 
+# Shut down Nova hypervisor plugins after Nova
+NOVA_PLUGINS=$TOP_DIR/lib/nova_plugins
+if is_service_enabled nova && [[ -r $NOVA_PLUGINS/hypervisor-$VIRT_DRIVER ]]; then
+    # Load plugin
+    source $NOVA_PLUGINS/hypervisor-$VIRT_DRIVER
+    stop_nova_hypervisor
+fi
+
 # Swift runs daemons
 if is_service_enabled s-proxy; then
     stop_swift