Support finding configuration information from other platforms or config structures

[rfdickerson]

Right now, CloudConfiguration relies very heavily on the conventions of VCAP_SERVICES. We should allow the configuration information to come from not only a path inside VCAP_SERVICES, and instead multiple paths that could be compatible with other platforms like Heroku, Amazon AWS, and our Docker deployment.

In other words, we should should support loading files with data that has been the result of something like this:

cf env MyApp > config.json

But often times, the user will want something even easier, especially the Docker case:

Some examples:

I think that a typical config.yml used for a Docker deployment could look like:

- port: 8080
- services:
    - postgresql
        - username: robert
        - password: pa55w0rd
        - hostname: postgresql.bluemix.net 

[rfdickerson]

Heroku I believe encourages using a web service that serves JSON to get the config variables. Therefore, we can just use your URLSession-based Configuration loader, @youming-lin . Still, not sure how to configure CloudConfig to point to that URL. It seems like the application would need to be self aware about what kind of environment it's running in.

https://devcenter.heroku.com/articles/platform-api-reference#config-vars