Evaluate new T3 burstable EC2 instance type for wpt-api.stage.mozaws.net

This should (and will, for tracking purposes) be also tracked in Bugzilla, but we should evaluate the new-ish EC2 T3[0] instance type.

Purported benefits: pretty much everything from the link will -- should -- benefit our instance(s).

/cc @jbuck @davehunt

[0] https://aws.amazon.com/about-aws/whats-new/2018/08/introducing-amazon-ec2-t3-instances/