Added support for specifying custom fields in /v2/info endpoint.

We have a number of features that we've added to Cloud Foundry (remote debug, remote console/JMX). Our client for these features determines the host it needs to connect by extracting them domain from the API URL and prepending a host name to the domain. This is what `cf logs` used to do.

We would like to do what `cf logs` does now by querying /v2/info to determine the endpoint our client should use to fulfill our custom features.

This PR adds support for adding custom fields to /v2/info. I don't know if "custom" is the right name to use but I couldn't think of anything better.

This change will also require adding an entry to the cloud controller job spec in cf-release. Such an entry could look like:

ccng.info.custom:
  description: "Custom values for /v2/info endpoint"