* Replace os_channel variable with os_image to align naming
across clouds. Users who set this option to stable, beta, or
alpha should now set os_image to coreos-stable, coreos-beta,
or coreos-alpha.
* Default os_image to coreos-stable. This continues to use
the most recent image from the stable channel as always.
* Allow Container Linux derivative Flatcar Linux by setting
os_image to `flatcar-stable`, `flatcar-beta`, `flatcar-alpha`
* Allow groups of workers to be defined and joined to
a cluster (i.e. worker pools)
* Move worker resources into a Terraform submodule
* Output variables needed for passing to worker pools
* Add usage docs for AWS worker pools (advanced)
