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?
I guess my hesitation comes from how many users seem to try Deis on unsupported versions of CoreOS already, which often fails, which is a headache, which is why we are so eager to land on stable and hopefully stop prescribing specific versions. It's not literally true that current Deis requires 494.3.0. But I fear saying "we recommend 494.3.0" leaves the door open for someone to try 445.0.0, which won't work. 

Saying "requires stable channel, v494.3.0 or later" represents something we can support, so perhaps that makes it a requirement? :wink: Seriously though, what about:

> Deis strongly recommends CoreOS stable [version 494.3.0](https://coreos.com/releases/#494.3.0) or later.