set up wp-config & its settings the way I like it

going to keep wp-config.php in the parent directory of where WP is
served out of, and the vars.php file is going to be stored outside of
source control, somewhere in the PHP include_path; any setting which
would reveal sensitive information about the site config has been moved
into vars.php, and it's set up in a way so there can be a master file
for the entire machine
Let WP-CLI know who the DB user is

In my usecase, I have WordPress sites using a DB user with the exact set
of DB permissions it needs, no more, no less, but I use a different DB
user as defined in my personal .my.cnf file