chore: re-enable dws-loadtest autoscaling with 1 cpu

What does this merge request do and why?

Sets:

• max_instances to 4 to re-able autoscaling
• cpu to 1000m to restrict the instance to a single CPU

This will let us check if single-cpu instances autoscale better than dual-cpu instances did

See gitlab-org/gitlab#578154

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.
• If this change requires executor implementation: verified that issues/MRs exist for both Go executor and Node executor or confirmed that changes are backward-compatible and don't break existing executor functionality.