Update dockerfile builds, and consolidate broker images.

[tobybellwood]

This tidies up a few services that are no longer needed:

• local-registry, local-dbaas-provider and local-mongodb-dbaas-provider aren't needed for a docker compose stack
• the broker and broker-single dockerfiles have been combined into one single image
• the configuration in the rabbitmq/broker image for clustering is no longer supported in rabbitmq (classic mirror) so has been removed from the image
• the git and data-watcher-pusher images are now built from commons instead of alpine for ease of update.