Enable autologin on VMs

What does this MR do?

Add note to enable Auto Login when building base OS images. Current OS images have been rebuilt.

Why was this MR needed?

What's the best way to test this MR?

Launch a VM with macos-{10.13,10.15,11}-90G-ssh.img. You shouldn't have to type your password.

What are the relevant issue numbers?

Closes #58 (closed)