Add support for Docker in Nova

Adds 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
Merge branch 'install-pip' into dt-test
Merge branch 'nova-plugin' into dt-test
Merge branch 'docker' into dt-test
Merge branch 'screen-stop' into dt-test
Merge branch 'doc-shocco-cleanup' into dt-test
Merge branch 'update-usb-tools' into dt-test