Add Docker registry

We are wrapping up the support for using Docker images and rFaaS (#10). However, we only use DockerHub, which has multiple restrictions - throttling and network latency. We want to locally deploy a Docker registry.

• Add scripts for deploying the Docker registry and generating JSON configuration.
• Add configuration of Docker registry in the rFaaS executor manager configuration.
• Add polling Docker images on container allocation in the execution manager - only when the image is not present already.
• Support registry credentials.