This role is for the (currently single) server that performs
rpm-ostree tree composes.  It takes as input two primary data sets:

1) The Fedora package set for a particular branch (f21, rawhide)
2) A git repository, https://git.fedorahosted.org/cgit/fedora-atomic.git

This output gets written into an OSTree repository which is pushed to:
http://dl.fedoraproject.org/pub/fedora/linux/atomic/

This is only for Fedora 21.  rawhide trees are handled via
https://git.fedorahosted.org/cgit/releng/tree/scripts/run-pungi

It uses lmacken's

https://github.com/fedora-infra/fedmsg-atomic-composer

which has the ability to listen for fedmsg notifications of package
set changes, but is not actually doing so at the moment due to
concerns about fedmsg reliability (?).