Multi-arch build orchestration

[jlebon]

This is a detail that was discussed a bit in #463 which I don't want us to lose. See specifically #463 (comment). As part of multi-arch enablement, we'll need some way to (1) farm out work for each arch to appropriate hosts, and (2) aggregate results. (Related: cosa's layout and commands already now support multi-arch).

Ideally, we'd do this from within OpenShift with node selection. Though with some effort, we could also just e.g. use Packet or some other cloud provider.

[cgwalters]

I created a higher level issue for this: coreos/fedora-coreos-tracker#262
Since we'll probably need work in the FCOS-pipeline and possibly other places too.

[dustymabe]

I created a higher level issue for this: coreos/fedora-coreos-tracker#262

That issue is closed. I'm going to close this.