Better, more configurable Docker support

[mattnappo]

This draft PR contributes to #10 and #19 by:

• Adding scripts to launch a docker registry with authentication (credentials)
• A rFaas base image Dockerfile (and scripts) to enrich a docker container with the rFaas executor binary
• Automatic image polling from a private docker registry instead of DockerHub. The new config/executor_manager.json configuration file serves as an example of how to configure the docker registry and container selection.
• Configurable, non-hard-coded Docker/Docker registry config for the Executor Manager.

Work that still needs to be done to wrap up Docker support:

• Docker network configuration scripts
• More testing of the scripts
• HTTPS registry support

Right now, the configuration system I added is capable of connecting to any registry. Of course, running a local registry on the same server as the executor manager is possible under this config.

[mcopik]

@mattnappo Thank you! This looks like a good start :-)

I have a similar but extended solution for container selection on a different branch (libfabric-sarus branch); maybe I can try to cherry-pick these changes to the development branch and merge them with your effort to get a nice interface for container type selection.

[mattnappo]

This has been moved to #29