* Use the official Kinvolk Flatcar Linux image on Google Cloud
* Change `os_image` from a custom image name to `flatcar-stable`
(default), `flatcar-beta`, or `flatcar-alpha` (**action required**)
* Change `os_image` from a required to an optional variable
* Promote Typhoon on Flatcar Linux / Google Cloud to stable
* Remove docs about needing to upload a Flatcar Linux image
manually on Google Cloud and drop support for custom images
* CoreOS Container Linux was deprecated in v1.18.3
* Continue transitioning docs and modules from supporting
both CoreOS and Flatcar "variants" of Container Linux to
now supporting Flatcar Linux and equivalents
Action Required: Update the Flatcar Linux modules `source`
to replace `s/container-linux/flatcar-linux`. See docs for
examples
