Integrate Docker with Software Config

A Heat Docker resource has been created which relies on the remote Docker api to create containers on compute instances:

https://blueprints.launchpad.net/heat/+spec/docker-plugin

This implementation will require certificates on the Heat engine and instance running Docker to be created for secure communication, which is not scalable.

Docker should be integrated into software config so that Docker running on the instance can listen on a unix socket, which will make using Docker within Heat more secure and scalable.