feat(*): provision using CoreOS stable channel

[CoreOS 494.3.0](https://coreos.com/releases/#494.3.0) was promoted to stable today, so Deis should be able to settle there. Provider image IDs & AMIs were all updated except Rackspace, which hopefully refreshes later today.

Double-check IDs at https://coreos.com/docs/running-coreos/cloud-providers/ec2/ and https://coreos.com/docs/running-coreos/cloud-providers/google-compute-engine/. Refs #2613 and #2631.

Question: is https://github.com/coreos/coreos-overlay/commit/f6ae1a34d144e3476fb9c31f3c6ff7df9c18c41c going to supercede our `--insecure-registry` settings?
@mboersma you are right.
How is possible to emphasize "stable channel" so with that you avoid any beta or alpha version (reading "Deis runs on CoreOS stable version 494.3.0 or later" I don't see anything wrong trying 699.3.0 beta)