Allow provisioning to Amazon linux

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

What critical bug this MR is fixing?

Technically none

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

This will allow to use AMZ linux instead of RHEL

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

When Gitlab Runner is deployed to AWS, using Amazon Linux is kind of obvious choice.

I tested provisioning with docker-machine command and it worked. Same when docker-machine is managed by Gitlab Runner process.

This MR should possibly close #43 (closed)