feat(*): provision using CoreOS stable channel

[CoreOS 494.4.0](https://coreos.com/releases/#494.4.0) 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? Answer: no, but we'll test it of course.
Correct answer: apparently neither one is applied? See https://github.com/coreos/coreos-overlay/commit/f6ae1a34d144e3476fb9c31f3c6ff7df9c18c41c#commitcomment-8833445