[atomic] how can we reuse the "inject yum repositories" functionality?

[Ken Dreyer <kdreyer redhat com>]

https://github.com/projectatomic/atomic-reactor has some functionality
to write Yum repo files to disk and add RUN lines to a Dockerfile to
enable these repos.

Specifically add_yum_repos_to_dockerfile() in
atomic_reactor/plugins/pre_inject_yum_repo.py

This method would be really useful to abstract out into a separate
reusable Python library.

My use-case is to inject Yum repos during a container build operation
outside of OSBS.

Right now in the rpkg-family tools, I can run:

  rpkg container-build --repo http://...

and that sends the buildContainer RPC to Koji, which interacts with
OSBS to inject Yum repos. This is all server-side, so it depends on
OSBS's capacity and availability.

It would be great to have an equivalent command that can do a "local
container build" operation, like:

  rpkg local-container-build --repo http://...

That would replace the FROM line, insert the appropriate Yum repos,
and use "docker build".

- Ken