Add support for Docker in Nova

* Add basic support for hypervisor plugins in lib/nova_plugins
* Add lib/nova_plugins/hypervisor-docker to use Docker as a Nova
  hypervisor.  This assumes that the Docker daemon is already running
  before stack.sh starts and aborts if the daemon is not found.

Change-Id: I9e7065b562dce2ce853def583ab1165886612227