Add support for Fedora CoreOS

⚠ ️NOTE THAT THIS FORK IS MAINTAINED FOR CRITICAL BUG FIXES AFFECTING RUNNING COSTS ONLY. NO OTHER CONTRIBUTIONS WILL BE ACCEPTED. ⚠️

Hi!
Fedora CoreOS was our OS of choice for the deployment of GitLab Runners when it came to the replacement of CoreOS.
Seeing that others might opt for the same OS, which is currently not supported by docker-machine out of the box, they might find these changes useful.

What critical bug this MR is fixing?

Adds support for Fedora CoreOS, which is the official successor to CoreOS, whose EOL was reached in May 2020.
Fedora CoreOS has been mentioned as a potential first choice in plenty of discussions:

• During the search for CoreOS' replacement - https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/9132
• During the initial assessment of Container Optimized OS - #14 (closed)

How does this change help reduce cost of usage? What scale of cost reduction is it?

In what scenarios is this change usable with GitLab Runner's docker+machine executor?

Related to #32