Switch autoscaling Docker Machine examples to GCP and Ubuntu
What does this MR do?
Closes #26955 (closed).
Updates the Docker Machine autoscaling examples for Google Cloud instead of Digitalocean as well as removes a couple of references in the docs to the now deprecated CoreOS and replaces with Ubuntu 18.04.
Why was this MR needed?
CoreOS was deprecated earlier this year and is now being fully removed from cloud platforms. We now use GCP internally and have working examples against that cloud service for Docker Machine so replacing with a known working example.