Support multiple GCE zones?

[solarce]

Currently we only support a single GCE zone in the config, https://github.com/travis-ci/worker/blob/master/backend/gce.go#L159, should we add support for multiple zones or just wait for cloud-brain?

[sarahhodne]

I think the best way to do this would be similar to how it's gonna work with cloud-brain: A pool of workers for each zone. So we have one pool of X workers that have a config for one zone, and a pool of Y workers than have a config for another zone. That way we don't have to keep track of which instance is associated with which zone.

[meatballhat]

This has been addressed another way 🎉.