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