Support updating from local mirrors

[bgilbert]

Many Fedora CoreOS nodes will update directly from our Cincinnati server and ostree repo, but there are two other use cases we should support:

1. Nodes updating from a local mirror on an internal network. The mirror has access to the Internet but the nodes may not.
2. As above, but the mirror doesn't have Internet access either. Updates are shuttled across an air gap.

We'll need tooling and docs for each.

[lucab]

I'm starting to add notes here, so we can more easily collect them into docs.

To use a customized/mirrored updates backend, at least all of the following topics have to be covered:

1. running an rpm-ostree mirror, and how to mirror/proxy ostree content for our releases
2. running a Cincinnati backend, and how to mirror/proxy our update-graph
3. overriding the default repository in rpm-ostree on each node
4. overriding the default Cincinnati endpoint in Zincati on each node