Add Vagrantfile.local & Vagrantfile.local.d, allowing user to override memory, cpu, etc.

Summary:
Recently, I bumped the memory allocated to Vagrant to 1024 MiB.  That's
way too much for some people's poor laptops.  Allow local overrides via
Vagrantfile.local or Vagrantfile.local.d/*.

Includes an example.

Test Plan: Manual testing

Reviewers: khamidou

Reviewed By: khamidou

Differential Revision: https://review.inboxapp.com/D485