Add unlimited EC2 instance option

There's a new feature for t2 instances, called "T2 Unlimited" (details can be found here: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/t2-unlimited.html)

It allows instances to sustain high CPU performance for any period of time whenever required. If you have a lot of jobs queued up and a constant load on a slave this option would speed things up as slaves would not exceed their burst hours and slow down.

Please implement an option for the slave to start as unlimited instance.