Add support for cloudfoundry CUPS and continuous integration

[toolness]

In 18F/ED-API-Program#14 I set up the AUTOAPI_HOST environment variable for a deployment of autoapi. However, if we move to continuous integration then the environment variables set by cf set-env will be wiped away. To remedy this we should use a Custom User Provided Service and bind it to the app, and document the mechanism for doing this.

For reference, see https://github.com/18F/calc/pull/310 for another Python project's implementation of this functionality. (Though we use cfenv which might make things easier.)