New AWS S3 bucket for software distribution

Similar to #1138 (closed).

Since Docker Machine is not maintained actively anymore and our current autoscaling relays highly on it, we've decided to maintain our own fork with the fixes, that we've created. For that we need to store somewhere compiled binaries.

As part of gitlab-org/ci-cd/docker-machine#4 (closed), we need to create another S3 bucket (let's say gitlab-docker-machine-downloads) where we will upload the binaries.

As in #1138 (closed), we want to use the same ACCESS/SECRET keys to authenticate, as we already use for accessing the gitlab-runner-downloads bucket.