Good morning,

I understood your app scenario:

+ Your app has to interact with CF API but your question is about where you can store your configuration.

I think that it is very normal to use a config.json to configurate your App. If you don't like to put parameters in a plain file (e.g. config.json), another alternative is the usage of manifest.yml file.

Read this links:
http://stackoverflow.com/questions/17902803/how-do-you-set-environment-variables-on-cloud-foundry-v2
http://docs.run.pivotal.io/devguide/deploy-apps/environment-variable.html
https://docs.cloudfoundry.org/devguide/deploy-apps/manifest.html#env-block

Juan Antonio