Add a new hypervisor to support Docker containers

Docker is an open-source engine which automates the deployment of applications as highly portable, self-sufficient containers which are independent of hardware, language, framework, packaging system and hosting provider. Containers don't aim to be a replacement for VMs, they are just complementary in the sense that they are better for specific use cases. Nova support for VMs is currently advanced thanks to the variety of hypervisors running VMs. However it's not the case for containers even though libvirt/LXC is a good starting point. Docker aims to go the second level of integration.