User management - fix auth attribute default

When I set `node['mongodb']['config']['auth']` to true in users.rb, I was inadvertently setting it to true even when `::user_management` wasn't included. I thought I was using include_attribute or, I don't know, what I'm doing here in this PR instead.

So that had the effect of making all mongo installs require authentication, even when no users were created. Quite a big problem. This should fix that so it's only set to true if the recipe is included, but still can be overridden to false by users.