Define / document topology API

[mojodna]

If I approach the topology codebase as a library and import topology, what are the public functions that I can call to do various things?

What do they expect as inputs? What do they provide as outputs?

This is necessary for wrapping the process in Celery.

/cc @joehand @brelsford

[cesandoval]

We are hoping to wrap the topology process in Celery with localco.de.