Support spot pricing on AWS

[milliams]

To make things cheaper, we should support interruptible instance type. This model is available on AWS as spot instances and Google as preemptible instance.

On AWS, an instance started as a spot instance will have a metadata service endpoint at http://169.254.169.254/latest/meta-data/spot/termination-time if it is marked to be interrupted. This should be polled for every 5 seconds.

This can then either be passed through the monitoring system so that the management node can respond or perhaps the compute node itself can trigger the POWER_DOWN state in directly Slurm with an scontrol call.

On the other side, we need a mechanism for specifying that the user wants spot pricing. Either this could be done on a cluster--wide level, a instance type-level or perhaps a job-level.