Describe the bug
The autograding worker machines are running at least a few unnecessary programs.
apache2, php-fpm, and postgres are unnecessary.

There may be other programs/utilities that are unnecessary.

NOTE: fortunately, these don't seem to be compute intensive and directly problematic at this time.

Expected behavior
If these programs are not installed out-of-the box from Ubuntu, we should not install them.
These programs/services are not needed and should not be launched.
I think we are just missing a wrapper "if not worker" around some of these tools in our installation/update scripts.

NOTE: Before making a PR fixing this, the edits must be sufficiently tested -- you need to have at least 2 vagrant VMs, one primary, one worker.