New metadata file that supports the new 'load_var' Concourse statement

[bgandon]

Hi there, This PR is meant to provide a solution to the issue #248. Here I've added a new metadata-map.json in .git/resource directory. This file contains the same information as the legacy metadata.json, but properly organised in a key-value format. Such JSON file can be loaded directly in the pipeline variables context using the new load_var step. See https://concourse-ci.org/load-var-step.html for more information. I've updated tests, they are running fine, and I've documented the new feature. Don't hesitate to provide feedback so that we can move on quickly. Best, Benjamin