feat(aws): allow swap on nodes

By default, kubelet does not run on nodes with swap enabled.

However, having swap can be useful, and we've used it successfully to handle spikes in memory usage, and thus avoid OOM-killing of build jobs. This change allows swap unconditionally, but we could also be conservative and make it conditional (option in the config).

Note that this does not actually enable swap. We use it in combination with node init scripts (!39) to detect if our ec2 instance has a local disk, and then enable swap if it does.