Python Support

I think this should be a story instead of an issue.

The description dives right into how to do this, but skips over what the goal is. :) "Lazy integration" is perhaps obvious to some, but it's helpful to have a more full description. Start with the desired behavior: we want the ability to set the download policy to on_demand or background. Describe the workflow of specifying that setting, accessing content, having it retrieved on-demand, etc. What will QE do to verify this?

The checklist is a good place for implementation details, including: add catalog entries during sync, document new importer settings, make new importer settings available in pulp-admin, add release notes, etc.

The "downloaded" boolean is already on FileContentUnit in platform, so you'll get that for free.

I'm not sure about the idea of updating the unit in the DB after download. Pulp's job is a LOT easier when units are immutable. Do we really need to update it for some reason?

PR:
https://github.com/pulp/pulp_python/pull/237