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.
not necessarily required insomuch as "recommended" due to docker 1.3 silliness