This adds support for launching Amazon EC2 Spot instances
(https://aws.amazon.com/ec2/spot/), which comes with huge cost saving
opportunities.
Amazon EC2 Spot instances are spare Amazon EC2 capacity, you can get
with an discount of up to 90% compared to on-demand pricing.
In contrast to on-demand instances, Spot instances can be relaimed with a
2 minute notification in advance
(https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/spot-interruptions.html).
When :attr:`providers.[aws].pools.labels.use-spot` is set to True, the AWS
driver will launch Spot instances. If an instance get interrupted, it will be
terminated and no replacement instance will be launched.
Change-Id: I9868d014991d78e7b2421439403ae1371b33524c
36dbff84ba