Add a setup check for installation on a burstable instance type
Summary: Fixes T11544. Attempt to detect if we're on a tiny, burstable-CPU AWS instance and complain.
Test Plan:
- Completely faked this locally.
- Hit the URI on an EC2 instance to check that it's correct (got back "m3.large", since that was the instance class).
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T11544
Differential Revision: https://secure.phabricator.com/D17014