Merge admin & gitlab-runner users

What does this MR do?

Merges the admin and gitlab-runner users into a single gitlab user.

Add detailed instructions on how to build base OS images.

Why was this MR needed?

In autoscaled VMs, we will connect through SSH (currently allowed by admin) and execute builds (currently done by gitlab-runner) using a single user, so we needed to merge the two.

What's the best way to test this MR?

Launch a VM using mr-60-runner-11-12.4.img and verify that you can SSH to it as gitlab

What are the relevant issue numbers?

Closes #55 (closed)